Charge for the guns!" he said. (line 6)
This is the soldiers' job. As crazy as it sounds, when the commander tells the soldiers to charge, then they just go and do it, even when it means almost certain death. The reality of duty is one of the big themes in this poem. Tennyson is definitely saying that these men deserve to be honored for their willingness to sacrifice, for this sense of responsibility that they feel.
Not though the soldier knew Someone had blundered (lines 11-12)
There's no question that the speaker believes in duty, in the importance of soldiers doing what they had promised to do. Still, he goes out of his way to suggest that someone, somewhere, has screwed up. Big-time. The speaker won't directly accuse the commanders, but he does say that this charge is only happening because someone "blundered." Now that doesn't add up to some kind of criticism of war in general, but we sure detect a hint of bitterness, a sense that the guys who boss these soldiers around aren't worthy of the sacrifice they are asking for.
Theirs not to make reply, Theirs not to reason why, Theirs but to do and die. (lines 13-15)
This famous triplet (three rhyming lines in a row) is a great, simple summary of the sense of duty these men feel. They don't question the order, they don't ask why, they just charge in, fight, and die. This might make you admire how steadfast and courageous these men are, but it might also make you a little upset too. These men don't have any of the basic freedoms that we expect in our daily lives. When someone says they should go get slaughtered, they just have to go. Then again, maybe that's just the way war needs to be, and we need brave people willing to do what it takes. It's a big question, and we don't think Tennyson has an answer.