Steem Power is your "influence" on the platform, basically the higher your steem power the more your upvotes on other peoples comments and posts are worth.

When you make a post or a comment then you are the author, so the rewards you get on your own comments and posts will come up in the author rewards section.

Once you have high steem power you it might be worthwhile to resteem other users posts. I believe that you can earn curation rewards for resteeming other peoples content up to a maximum of 25% of the total payout of that post.

Hope this helps you :)