Was there a man dismayed? Not though the soldier knew (line 10-11)
You just can't keep the Light Brigade down. Apparently not a single one of the six hundred of them feels discouraged ("dismayed") at all. That's maybe a little hard to believe. We bet there were at least a couple of guys in that brigade who really wished they were somewhere else. Still, we can allow Tennyson a little exaggeration to make his point. We think line 11 is important. These guys aren't brave because they're stupid or ignorant. They know that they're facing almost certain death, but they charge anyway. Now that takes real guts.
Theirs but to do and die. (line 15)
This is a pretty terrific boiled-down expression of the courage of the Light Brigade. They just "do and die." Two little words that say a lot in a little bit of space. That simple, elegant phrasing has gone a long way toward making these some of the most famous, quotable lines in 19th-century English poetry.
Boldly they rode and well, Into the jaws of Death, (lines 23-4)
Another neat way of showing us how brave the boys of the Brigade are. They ride "boldly" and "well," showing not just their guts but also their skill in the face of death. It's a neat little balancing act that Tennyson does here, contrasting the studly, fearless soldiers with the ugly, harsh image of the jaws of death. It helps to make the link between courage and tragedy that's such an important part of this poem.