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It means someone is lending you steempower. They can always take it back, but for now it is completely yours ☺️

Thank you :)

On what conditions take the witnesses the delegated steempower back. And what can one do with the delegated steempower. I'm new so I'm just trying to grasp how things work here.

Some may delegate steempower to new users. So, when a user is not so new, the giver of the steempower might take it back to distribute to a newer user.

As for what you can do with steempower, it boosts the rewards for curation, making your upvotes worth more.

Hope this helps.