The order is repeated. The speaker really wants us to focus on those words, on the command to move forward. The men are being sent to their doom.
Again, we don't know who's giving the orders here, but this disembodied voice might make us pause and think about why these brave men are being sent into "the valley of Death."
Was there a man dismayed?
Now we're trying to get a peek into the heads of these soldiers, trying to imagine how it must feel to charge toward death.
The speaker asks if any of the soldiers were "dismayed." In this case, to be dismayed means to lose your courage, to be overcome by terror or sadness. That would be a pretty normal reaction to a situation like this.
Not though the soldier knew Someone had blundered.
Of course the Light Brigade is too tough and loyal to feel dismayed.
That first word, "not," implies that these men don't feel discouraged at all. They're ready to do their job, even though the order might be crazy.
This is a really important point in this poem. The soldiers aren't dumb. They know this charge isn't a good idea, that someone has made a mistake, has "blundered."
This is as close as the poem gets to criticizing the men who ordered this attack. The speaker is no revolutionary, but we think you can feel some anger at the commanders simmering under this poem, especially at this moment.
Theirs not to make reply, Theirs not to reason why, Theirs but to do and die.
This is a famous group of lines, and for good reason. Do you see how they fit together, the way they share the same first word and the same rhyming sound at the end? Do you see how simple they are, too? There's no showing off, no fancy words (in fact almost all the words in these lines are one syllable).
The speaker uses these lines to sum up all of the honest, humble heroism of these men. They're just doing their job. That job doesn't let permit them to talk back to their commanders ("make reply") or to figure out the point of the attack ("reason why"). All they can do is to ride and fight and possibly die ("do and die").
Into the valley of Death Rode the six hundred.
These last two lines are the same as the last two lines in the first stanza. In poetry, that's called a refrain (like the chorus in a song). It emphasizes the main action of the poem, which is these men riding to their death. It also gives a smooth, dignified rhythm to the poem.