A blunder is a mistake, and a careless one. It’s caused by someone not paying enough attention or overlooking important details, and this is exactly what happened to the “Light Brigade” of the poem. Tennyson is basing his poem off of a real, recent event that caused an uproar in England as the result of a poorly made decision all the way in Crimea.
A light brigade is a cavalry unit, lightly dressed and lightly armored. They carry pistols with them, but primarily exist to cut down enemy combatants with sabers that they wear at their hips. Often a light brigade is meant to take smaller forces, to overwhelm infantry on the ground, or to act as a sort of clean up crew at the end of the fight, finishing off units as they break up. A light brigade is not made to oppose large scale artillery, but that’s exactly what they were called to do in the Battle of Balaclava in 1854, at a significant cost of life.
Tennyson’s admiration for the light brigade comes from brigade's courage in the face of the blunder. They knew someone had made a mistake in giving their orders, but it was not up to them to question orders, but to follow them. For Tennyson, this is both respectable and patriotic.