Can someone explain Steem Power and delegation in more details, please?

Hello Steemit!

As you know I am a new user. I registered on Steemit approximately 1 month ago.
I read a few guides on the site but I still don't understand exactly what the delegation is and how it is related to Steem Power?

Steem Power
"Influence tokens which give you more control over post payouts and allow you to earn on curation rewards."

Okay, I completely understand this.

"Part of your STEEM POWER is currently delegated to you. Delegation is donated for influence or to help new users perform actions on steemit. Your delegation amount can fluctuate."

This is the confusing part. What is the delegation in this case? How does it work? Is everyone else on Steemit have delegation?
Where does it come from? From other users? Or the system? Or both? How long will it last?
What does (+145.446 STEEM) means in my case?

Thank you very much for any help!

Enjoy Steemit!
Have a nice day!