"The Garden" is basically a poem about someone who thinks that hanging out in nature is the coolest thing a person could do, and being able to hang in nature by yourself is the whipped cream and cherry on top of that already delicious garden sundae. But the poem doesn't start out that way. In fact, the poem opens far, far, away from Marvell's garden world with a condemnation of society. It criticizes men who work their butts off to gain public recognition because really, they're losing out—all that time in the office means they aren't spending time in the great outdoors and, according to Marvell, they're got their priorities all mixed up.
Next the speaker talks about how he used to look for "quiet" and "innocence" in society but has discovered that even the best that mankind has to offer isn't as awesome as the solitude of nature, a point which is capped off by a cute little anecdote that explains why trees are better than girlfriends. And just in case anyone remains unconvinced, the speaker calls in some Greek mythology for backup—even classical gods like Apollo and Pan, he says, liked plants better than people.
Stanza 5 marks a shift in the speaker's argument from nature vs. society into pure praise for the awesomeness of nature, epitomized by the idea of "the garden." Wherever this garden is, the speaker is clearly pumped up about the idea of spending lots of time there. There's lots of delicious fruit, so many melons that you can't even find room to walk, and nice cushy grass to fall on in case you get too drunk on all the wine that "luscious clusters" of grapes are dripping into your mouth.
Marvell then takes things up a notch and starts talking about spiritual things like the mind and the soul. It's complicated language, but the speaker is basically saying that being in the garden allows the mind to be at its best and the soul to be its most content. And what would be extremely awesome for the mind and soul would be the opportunity to enjoy the garden alone; in Biblical terms, Adam would have been better off if Eve never existed (the jury is still out on what Marvell's wife thought about this last bit). The poem concludes with some images of God as a gardener-clockmaker and compares the flowers in the garden to a super-huge sundial by which humankind can measure their lives.