Power Up 100% = All your rewards will be Steem Power. Steem Power is the influence token, which makes your votes stronger and you can power down into Liquid Steem. Choose this if you're committing to Steemit long term, or want to grow your power.

Default (50%/50%) = Half your rewards will be Steem Power as above. The other half will be either Steem Dollars, Liquid Steem or a mix of the two. Those two are the tokens which you can sell on an exchange immediately for Bitcoin, which you can sell for dollars. Choose this if you want to make cash money from Steemit.

Decline Payout = You choose not to get any rewards at all from the post. There are times when you might want to do this, but generally you shouldn't choose this one.