Lines 21-33 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Then one hot day when fields were rank
With cowdung in the grass the angry frogs

Heaney has shifted from the collective memory of "every spring," when the speaker would collect frogspawn and their teacher would give them a lesson on the reproductive nature of frogs, to a different, unique and "hot" day. Is something big about to happen?
It's another stinker of a day by the flax dam. Cowdung (cow poop) is steaming in the grass—lovely.
And now the frogs are angry. It's like Angry Birds, only with less feathers and more goop and cow poop. Never before in the poem have the frogs had any emotion. This is a first, and the fact that it's coming at the beginning of the second (and final) stanza might be signifying a shift in action or tone (or both).

Invaded the flax-dam; I ducked through hedges
To a coarse croaking that I had not heard
Before. The air was thick with a bass chorus.

So the angry frogs are on the move. There's a whole army ("invaded" makes us think of armies) of them, and they're taking over the flax dam with their croaking.
The bass (low, loud sound) is something the speaker hasn't heard before, which is strange because he recalls that just about every year he collects the frogspawn. Perhaps this is later in the summer, after the tadpoles have grown into bullfrogs? That might be intimidating. Imagine hearing and seeing a whole mass of these guys.
Again, Heaney describes the air (something you cannot touch) as thick (a textural description). Remember the gauzy sound?
This description has the same effect. It's wrapping around the speaker, maybe even choking him a bit.

Right down the dam gross-bellied frogs were cocked
On sods; their loose necks pulsed like sails. Some hopped:

It seems like the speaker is getting a little skeeved-out by the frogs. Heaney describes them as "gross-bellied" and talks about their "loose necks."
The "loose necks" that "pulsed like sails" is a simile that describes how the frogs make their croaking sound, by puffing out their throats.
The frogs are cocked, as in pulled back ready for firing (like a gun). This is a somewhat violent way to describe the way the frogs are leaping from place to place. The speaker isn't exactly giddy with excitement like he was when he would scoop up the frogspawn. He's more appalled watching the bullfrogs.

The slap and plop were obscene threats. Some sat
Poised like mud grenades, their blunt heads farting.

Again, he's totally grossed out, even offended. The speaker perceives the sounds of their hopping around as "obscene threats."
They're readied like "mud grenades" and the sound of their croaking and slapping around sounds like farting. Bad times.
The speaker has gone from mildly grossed out, to offended, and now it seems like he's even a little afraid. The "grenades" are the third example of violent, threatening language used to describe these frogs, along with "invaded" and "cocked."

I sickened, turned and ran. The great slime kings

The speaker is so grossed out and afraid of the scene that he has to flee from it.
The "great slime kings" refer to the frogs. They're slimy because they live among the grime of the flax dam, and they're born from slippery frogspawn. The speaker thinks of them as metaphorical kings because they seem to dominate the landscape of the flax dam with their load croaks and slaps, and also because there are so many of them.

Were gathered there for vengeance and I knew
That if I dipped my hand the spawn would clutch it.

The frogs are not, of course, gathered at the dam for vengeance (violent revenge). At least, we're pretty sure they're not.
They're there because that's their natural habitat. But the young speaker, so intimidated by what he saw, thinks the frogs are out to get him.
Why him? Well, he admits to having scooped up his fair share of frogspawn in his day. He thinks all the big bullfrogs are gathered there to pay him back for robbing their young.
It's a funny way to end the poem. We know the frogs aren't going to "get" the speaker. He's probably being a little dramatic.
But he was certainly shaken by seeing those big, nasty bullfrogs gathered there, and will probably never look at frogspawn the same way again. (Neither will we.)
The transition from how the speaker sees the frogspawn to how he sees the result of frogspawn (the actual frogs) is a complete one-eighty. He's totally into the gooey frogspawn, and sees it as harmless, even fun. But when he sees the live and croaking bullfrogs, he's sickened and afraid. For the speaker, the joy of frogspawn has been forever spoiled by this traumatic experience.