Welcome! 100% power up is for Steem Power, which you can think of as 'influence' or 'engagement power.' 50/50 splits rewards between Steem Power and STEEM tokens that can be traded on exchanges for $. Here's a nice overview from Hive account@kevinwong for why accumulating more Steem Power may be preferrable to some people: https://steemit.com/steem/@kevinwong/why-should-you-consider-accumulating-steem-power

Good luck and have fun! ;)