The six hundred men of the Light Brigade were not dismayed, that is, disheartened or intimidated, by the awesome odds against them and the risks they were taking by riding recklessly into a barrage of cannon fire. This was because they were trained to obey orders without question. Not one of them questioned the order to launch an attack against far superior forces. The first line of the stanza is put in quotes to show that this was the order the riders were responding to. And as the rest of the stanza tells us:
"Forward, the Light Brigade!"
Was there a man dismayed?
Not that the soldier knew
Someone had blundered.
Theirs not to make reply,
Theirs not to reason why,
Theirs but to do and die.
Into the valley of Death
Rode the six hundred.
As stated in the "Analysis" section of the eNotes study guide for "The Charge of the Light Brigade":
Like good soldiers, they are willing to follow their leaders’ orders, even though they can see for themselves that the mission is sure to be suicidal.
The "Summary" of the poem in the eNotes study guide clarifies the meaning of the word "dismayed" and explains that the use of the question "Was there a man dismayed?" was intended to cause the reader to think about the soldiers' bravery before the poet answered.
Although we currently closely associate the word "dismay" with "shock," its actual meaning includes a loss of courage. By raising this issue as a question and then answering that no, there was no fear, Tennyson gives the reader a moment's pause to let the full extent of the soldiers' bravery sink in.
Alfred, Lord Tennyson held the prestigious position of Poet Laureate in 1854 when the charge was made at Balaclava. It was appropriate that he should publish such a patriotic poem. The rhythm of "The Charge of the Light Brigade," along with the irregular lines, is intended to suggest the galloping hoofbeats, the bursts of cannon fire, and the flurry of shells causing horses and riders to plunge to the earth. The British Empire was at a high point in the middle of the nineteenth century, and British soldiers were widely admired for their courageous, and sometimes futile, stands against far superior odds.