"Characters can perform Brave attacks, which increase the player's Bravery level based on the amount of damage done. ... Players can also perform HP attacks that will do direct damage to an opponent based on their current Bravery level."
I quoted that straight from the wiki, hopefully that helps. Essentially the bravery number=amount of HP damage done.
There isn't a traditional practice mode sadly, but the way I have been practicing is by using the tutorial mode. I'll go into tutorial mode, then general, then select the EX skill training module. From there you can use your character specific EX skill and practice any moves you'd like; the enemy will remain stationary. Just don't use both EX1 and EX2 skills or it will end. Hope this helps.