Thats the option you have on how you want to get paid out by Steemit. If you select the 50%/50%, all the rewards for that post will be shared 50% of rewards go to you, 50% to others who comment and upvoate and sll that good stuff, and this you get paid in steem dollars (which roughly are close to 1 dollar)
if you click 100%, you get 100% of the author rewards but they put it all into steem power for you. steem power is what ultimatley affects how much your upvote is worth, the more you have the more its worth.
its a general thought that if you click the 50/50, you stand to make sliiiiightly more money, however the 100% poeer option will ultimatley make your votes worth more. I always switch between them until i csn determine myself which is really better.
cheers