The soldiers in the Light Brigade are being "stormed at," by gunfire, an image that picks up on the word "thundered" in the line we just read.
The "shot" (bullets) and "shell" (big explosives fired from cannon) are a violent, noisy, destructive force that reminds the speaker of a storm.
Boldly they rode and well,
These guys aren't scared of some gunfire, though. In fact, they ride "boldly" (bravely) even though this is looking more and more like a suicide mission.
The point of this poem is to show us how heroic these men were.
Into the jaws of Death,
Tennyson has a lot of images for this scary valley, and he brings some more of them in here. Now the valley of Death becomes the "jaws of Death."
We'll admit it's not a super-original image, but it works well here. It's almost as if these guys were riding into the mouth of some kind of ferocious animal.
Into the mouth of hell Rode the six hundred.
This is the spot (at the end of the stanza) where the refrain belongs (see lines 7-8 and 16-17), but Tennyson switches things up a bit here. Instead of "Into the valley of Death," now the men are riding "Into the mouth of hell."
The "mouth of hell" matches up nicely with the "jaws" in the line before, and it's just one more way of emphasizing how bad the valley is and how brave these men are.
Changing the refrain also helps to keep us on our toes a little, and keeps the poem from seeming stale or repetitive.