All year the flax-dam festered in the heart Of the townland; green and heavy headed (1–2)
In the center of the town is a flax dam, where nature flourishes come springtime. "Heart" is a deliberate word choice here, and it's got nothing to do with Cupid. Heaney wants to get across that this place in nature is important in this poem, and it's particularly important to our speaker.
There were dragon-flies, spotted butterflies, (6)
The flax-dam is buzzing with flies and butterflies. Wildlife abounds in this tiny ecosystem and our speaker is like a kid in a candy store—totally in awe, and totally stoked.
But best of all was the warm thick slobber Of frogspawn that grew like clotted water (7–8)
The dam water is home to more than flax. Along with the flies and butterflies, frogs are sprouting up all over the place too, and our speaker couldn't be more psyched about this natural bounty. What's better than, uh… "warm thick slobber"? (Never mind, don't answer that.)
The fattening dots burst into nimble- Swimming tadpoles. (14–15)
Heaney gives us a mid-poem biology 101 lesson on frog reproduction just to show how invested our speaker is in this aspect of the natural world.
[…] You could tell the weather by frogs too For they were yellow in the sun and brown In rain. (19–21)
This is an instance where you can really see that the speaker pays attention to, and is excited by, what's going on in the natural world. You can almost picture him housing Lunchables and spouting these fun facts to his friends in the school cafeteria.
Then one hot day when fields were rank With cowdung in the grass the angry frogs (22¬–23)
Our speaker continues to explore his natural surroundings. Down in the hot, stinky fields, the speaker spies the frogs croaking away. This is the point in the poem where the speaker's relationship to nature starts to shift. We smell trouble in paradise.
Right down the dam gross-bellied frogs were cocked On sods; their loose necks pulsed like sails. Some hopped: The slap and plop were obscene threats. Some sat Poised like mud grenades, their blunt heads farting. (27–30)
The speaker observes how the bullfrogs are naturally, and his opinion about frogs starts to change—he's overwhelmed by their hopping and grunting, and begins to feel disgusted by them, and afraid. Head farting, in this case, is no laughing matter (we know, it kind of is).
I sickened, turned, and ran. The great slime kings Were gathered there for vengeance and I knew That if I dipped my hand the spawn would clutch it. (31–33)
The speaker pretty much experiences a one-eighty in terms of how he feels about the frogs in the flax dam. In the beginning he's bursting with enthusiasm, but by the end he's completely grossed out. He's even afraid that his interference (collecting the frogspawn) would be offensive enough to these mean looking frogs that they'd try to pay him back. Who knows what Revenge of the Frogs would look like, but he's not sticking around to find out.