Stanza 5 Summary

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

Cannon to right of them,
Cannon to left of them,
Cannon behind them
Volleyed and thundered;
Stormed at with shot and shell,

Feeling a bit of déjà vu? You should be, because these lines are almost an exact repeat of the beginning of the third stanza (lines 18-22). The only change is in line 41. The cannon that were in front of them are now behind them, which means that the Light Brigade has turned around and leaving the enemy behind them.
The return trip is just as deadly and terrifying, it's just turned around.
Even though he's describing really awful stuff, Tennyson still manages to give this poem a nice feeling of balance. Repetition is an important tool that helps him achieve that effect.

While horse and hero fell.

As we learn about the retreat from the charge, the poem emphasizes the loss of life.
Here we get an image of horses and soldiers collapsing under the rain of gunfire.
Notice also that this is the first time that the speaker comes out and calls these men heroes, although that's clearly been the message from the beginning.

They that had fought so well
Came through the jaws of Death,

Now, a part of the Light Brigade returns back to safety, after having "fought so well."
At the beginning of the poem we heard about how they were going "Into the jaws of Death" and now they are coming out again. In a way, it's almost like watching a movie played backward. They charge forward…they charge back. They run into the mouth…they run back again.
Of course the big difference is that there are a lot fewer of them now.

Back from the mouth of hell,
All that was left of them,
Left of six hundred.

This stanza ends with the words "six hundred" just like all the others did.
In this case, though, the tone is much darker, and the emphasis is on how many men have died. The speaker doesn't say how many make it to safety, but we're guessing that it's a small number.
That's the final image we get off the battle itself, the remnants of the Light Brigade moving back across the field.