First of all, let's get some vocab out of the way. Flax is a type of plant that grows annually (once a year). It grows from a seed, it's edible (get your Omega-3 on), and has blue flowers. In addition to being edible, flax is often grown for its fiber, which can be used to make fabric.
A flax dam isn't actually a dam, but a muddy patch of earth that's soaked to soften the flax. During this process, things get pretty stinky. The plant basically rots as it softens, letting off an unpleasant smell. Blech.
Also, "fester" means to decay or rot. When it's used to describe a wound, fester means to get infected with pus, or turn septic. Heaney is making use of both definitions in this first line. He literally means that all year the flax dam rots, but because the line ends with "heart" he can play around with the second definition, too (as in: the flax dam is like a rotting wound). Either way you look at it, it's a gnarly way to open a poem.
Of the townland; green and heavy headed Flax had rotted there, weighted down by huge sods.
That clears the festering heart image up. If you piece together the first line with the second, you'll see that Heaney means that the flax dam is rotting in the middle (the heart) of the town.
Our speaker begins to describe the rotting flax: green and heavy on top, weighed down by large sods ("sods" are pieces of cut turf, and in this case, they help cultivate the flax).
When the speaker chooses to describe the flax as "heavy headed" rather than, say, top heavy, he's giving the flax a human quality. This, along with the festering wound, makes the flax dam come alive a little bit—in a gross and smelly way. It's not a stagnant plant; it's a living and changing thing that fascinates the speaker.
So there is a flax dam in the middle of the town, and during a certain time of year, it really stinks up the place. Good times.
Daily it sweltered in the punishing sun. Bubbles gargled delicately, bluebottles
The dam's rotting is helped along by the hot sun. Think of garbage piled up in a city street—it's much smellier in the hot months than in the winter.
It's very likely spring or summer. It's hot and steamy, and the sun is burning down on the flax.
The bubbles in the wet earth are letting off a gargling sound. It's interesting that Heaney describes it as "delicate," considering that the rest of the scene so far seems pretty gross. It's a little bit of contrast added to the description to make the scene more vivid.
Bluebottles are a type of fly. They are presumably buzzing around the flax-dam like flies might buzz around garbage.
After reading a few lines you might have noticed a recognizable rhythm in them (don't worry if you didn't—that's what we're here for!). Heaney writes this poem in blank verse. We discuss what that means in detail in the "Form and Meter" section, but for now you just need to know that blank verse makes for smooth and consistent beat in each line. It's the musical glue that holds it all together.
Wove a strong gauze of sound around the smell. There were dragon-flies, spotted butterflies,
The speaker describes the buzzing of the bluebottle flies as a "strong gauze of sound." Gauze is a thin, loosely woven fabric, something that we usually associate with giving a feeling or appearance, like any physical thing, and not really having any sound. But, clever guy that Heaney is, he takes the properties of gauze—its thinness, its ability to wrap (as a fabric or bandage)—and attributes it to the way the flies' buzzing sound is wrapping around the flax-dam. This is called synesthesia, or using one sense to describe another. So, perhaps he's saying the sound, though thin, is so concentrated in one area that you can almost see it. Or, maybe more realistically, he's describing what he sees—the hordes of flies buzzing around that look gauzy—and rightfully associating that image with the sound. It's an effective mash-up of the senses and helps illustrate just what the speaker is witnessing.
Gauze is also something that we typically use to wrap around wounds. Don't think Heaney could slip this one past us unnoticed! After all of that talk of festering in the first line, we've got wounds on the mind—well, not literally (ew). Describing the flies' buzzing as gauze reinforces the wound image. We can see it even clearer now.
The description of the scene continues in a straightforward way, too: there were dragonflies in addition to the bluebottles, and there were spotted butterflies as well.
The speaker's descriptions so far weave both the pretty (the delicate gargling sounds, and the spotted butterflies) with the ugly (the festering dam, the rot, and "smell"). As readers, this can make the picture realistic for us—not every scene is picture perfect, or completely ugly—and it also gives us a sense that the speaker in the poem might have conflicting feelings about the scene, too. Let's read on to see if we're on to something…
But best of all was the warm thick slobber Of frogspawn that grew like clotted water
Science lesson: frogspawn is how frogs reproduce. It's jelly-like and fed by the sun, and turns into tadpoles (then eventually frogs). Gross and fascinating, right?
It's funny that the speaker says "best of all" when he's describing something so yucky. (Now, we're just assuming the speaker is male since we have no evidence to the contrary.) He says it's like "warm thick slobber," which is reminiscent of dog slobber.
But some people, particularly young people like the speaker at this point in the poem, can be kind of into gross stuff. It's just as fascinating as it is nasty to a young, curious mind.
"Clotted" means lumpy and coagulated (stuck together). So it's this clumpy, jelly-like mass that's baking in the sun in the flax.