Steem power is basically your level of clout in the community. The more steem power you have, the more your upvotes and stuff are worth.
Steem is the main currency in the steem it world. It fluctuates up and down with the market. When I started, it was worth 10¢. Around a month or two ago, it was up to $2.80. Today, a steem token is worth around $1.40.
Steem Dollars (SBD) is a strange one. It's like another currency that's more stable and stays close to the value of $1.
Like it was mentioned above, 100% power up means all the rewards for that post will go right into your Steem power. 50/50 means you're rewarded in half Steem Power and half Steem dollars.
Basically, power up is good if you want to build up your clout and reputation and are in it for a longer investment. 50/50 is good if you want to cash out your rewards.