This is an old video, but it'll give you the basic concept of it.

https://steemit.com/steemit/@shredlord/steem-steem-dollars-and-steam-power-explained

Here are some things I want to emphasize other than the video itself...

Steem and Steem dollars can now be trade into Bitcoin, bitcoin cash, Ethereum, Litecoin...and vice versa...

The more steem power you have, the more influential you are on Steemit.

Steem can be used to power up and exchange for steem power and that will increase your influence (voting power, getting more exposure)

Steem Power can be used to power down and exchange for Steem which you can exchange for real usable currency, but you will lose power and influence. It is recommended that you have at least 25 Steem power to keep the blog going.

When you vote, you will give away SBD/SP. When you get votes, you will noticed that you will receive SBD/SP. Be careful though, the more you vote, the less power you have. To keep track, go to this link https://steemd.com/@swiimward

Please note that the video is a bit outdated but informative. Also know that I am still a minnow myself, so take my advice with caution.