This is Steves neutral B. It has three uses. It can be used to mine for later use, craft better weapons each with its own specialty, and place blocks down depending on what materials you have mined. You can craft weapons from wooden, stone, iron, gold, and diamond. Gold is the fastest but also is easy to break. Diamond is the best for damage dealing and for its sturdiness.
Steve's side be is the minecart. Allowing for him to travel side to side dealing damage if hit. It can also grab an opponent if jumped out of. In order to escape you need to mash. Mashing will vary depending on the percent.
The up b of steve is the Elytra. Having a gliding like property it allows Steve to recover. Depending on how used can get you out of sticky situations.
Steve's down b places a block of tnt. Pressing down b while moving allows you to place redstone and eventually a pressure plate. Step on the pressure plate and the block of tnt explodes. Use it to ledge trap to limit options or trap characters in the minecart leading to the pressure plate.