Welcome! Glad to see you here.
Steem Power is directly linked to your voting weight. The higher it is the more weight your vote is worth and potentially more money you award to someone else by upvoting. You power it up by converting Steem to Steem Power. I'm not sure how powering down affects vote weight, but if someone is in a constant state of powering down it signals to me that they are not really invested in the platform. They look like they're the take the money and run type of user.
Here is a little about how Steem SBD & Steem Power works:
https://steemit.com/steemit/@donkeypong/still-confused-by-steem-steem-dollars-and-steem-power-the-power-plant-analogy
It's hard to find good guides. Here's a couple links for you.
https://steemit.com/steemit/@donkeypong/simply-steemit-your-no-nonsense-guide-to-getting-started
https://steemit.com/steem-help/@liberosist/mind-your-votes-an-investigation-and-guide-to-maximizing-your-curation-rewards
Thank you so much for this. It is really helpful and I appreciate your time in posting these links. Phew... :-)