RE: A lot of Plagiarism on Steemit

Flag is just an upvote but the other way around ;-)
Being serious now, under the hood it looks like this:
Every user has their own Steem Power which determines impact on post's potential payout.
You can decide to apply that with range from -100% to +100%.
On a web page, for beginners, it's usually limited to simply upvote is +100% vote, flag is -100% vote
Users with bigger amounts of Steem Power can fine-tune those amounts (i.e. give +1% vote which I'm going to do with your comment just now)