You write stuff on the site. If people like what you write, they vote for your content and you earn tokens based on the Steem Power of the vote.

When you earn enough tokens, you can POWER UP and give out bigger votes. At first, your votes will be worth nothing, but slowly they will be worth more as your own Steem Power grows.

https://en.wikipedia.org/wiki/Steem has all the information you could ever need about Steem.