Hey! I'm still learning too, but if we all help each other we'll get there!
I tend to only upvote most of the time, it's hard to think of something to say sometimes.
What that means is you have 12.3 Steem Power, and Steemit has sort of loaned you 2.7 SP to keep you around 15SP total. When you have 15SP of your own, Steemit will take back the loan. It's to make sure even new users votes count for something. Does that make sense? If you look at my wallet, I have none given to me, but a new user will have around 14.5SP delegated to them.
As for upvoting your own comments, it's up to you. If you want your comment to appear higher in the comments section then upvoting will help that. I think it's fine when you're a small user, although I don't do it generally myself. I hope that helps! Peace!