Could someone please explain to me how does SteemPower work :) ?

Hello, I am a new (french) steemit user. I am trying as best as possible to understand all the features of Steemit and how does it works. And I still get hard do understand some fiew things, like how does SteemPower work?

I saw that when we post, we can choose to earn half steem tokens / half steem power, or only steem power. So I choose to only earn in steem power, i guess that is to grow in popularity. But how does it really work ? How do we count it? What does having 1/10/100 or 1000 Steam Power means ?

Thank you in advance for your help and for giving me time by answering :) !