100% power up means all of your shares of the earnings goes to powering up your account with Steem Power.
50/50 means you want some (half) to go to Steem Power and some (half) of your earnings from the post to go to your money account (either Steem or Steem Dollars) that are readily available for trade or to pay for stuff.
RE: Power up 100% or Default 50% 50%. What does it mean.