Charging an army, while All the world wondered. (lines 30-31)
Tennyson clearly expects us to be impressed by these guys. Well, who wouldn't be amazed by a bunch of guys on horses "charging an army"? This David and Goliath image (the underdog fearlessly taking on the impossible challenge) is meant to astonish Tennyson's readers, to fill us with awe. He imagines that "all the world" will be amazed. Suddenly the audience for this desperate charge is not just the people standing there on the battlefield, but the whole world, everyone who reads this poem and hears this story.
While horse and hero fell. (line 44)
Here Tennyson finally comes out and says what he's been implying all along: these men are heroes. In this moment, when it all comes to a sad and bloody end, we really feel their bravery. Heck, it's hard not to feel sort of bad for the horses, too.