Flagging is the least effective way of affecting the rewards pool allocation.

"What is a flag?"

It is actually an upvote... but the other way around ;-)
Much in the same way as you get an upvote, which is by default a 100% vote, you can get a flag, which is a -100% vote.

"Why did someone flag my post?"

There could be many reasons.
Here are just some of them:

• Your post is in a wrong category or you have used wrong tags, or at least the user who flagged your post thinks so.
• Your post is not yours (plagiarism), or at least the user who flagged you thinks so.
• Your post is considered spam, or at least the user who flagged you thinks so.
• Your post is overvalued, or at least the user who flagged you thinks so.
• Your post is undervalued, but the user who flagged you does not know how to upvote.
• Your post is so bad that another user could not resist and flagged you.
• Your post is so good that another user was jealous and flagged you.
• Your post contains photos of cute kittens.
• Your post does not contain photos of cute kittens.
• The user who flagged your post has a secret reason.
• The user who flagged your post does not have any reason.
• The user who flagged you has a bad day.
• The user who flagged you has a good day and loves flagging others.
• The user who flagged you does not like you.
• Nobody likes the user who flagged you.
• Nobody likes you, but the user who flagged you was the only one honest enough to flag you.
• The user who flagged your post is @asshole or just an asshole.

Users do not have to upvote your posts, users do not have to have a reason to upvote your posts. Much in the same way, users do not have to have a reason to flag your post.

"What should I do about it?"

Nothing.
Unless you believe that someone had good reason to flag your post.
In that case, you should change your behavior.
Unless you do not want to.
Then do not.

"You said that flagging is the least effective way of affecting the rewards pool allocation. So what is more effective?"

RTFM for Steem’s sake!
Upvote valuable content.

---

_{(*) "Fun with Flags" is an online vodcast featured on The Big Bang Theory which Sheldon Cooper hosts on the subject of vexillology, the study of flags. Picture from the show.}