What does this mean?
The amount of rep (points) on your account has decreased by one.

Why did you do this?
There are several reasons why I -rep a profile:
‏‏‎ ‎‏‏‎‏‏‎ ‎ ‏‏‎ ‎‎•‏‏‎ ‎‏‏‎ ‎Rudeness towards other gamers,
‏‏‎ ‏‏‎‏‏‎ ‎ ‎‎‏‏‎ ‎•‏‏‎ ‎‎‏‏‎ ‎Spreading incorrect information,
‏‏‎ ‏‏‎ ‎‏‏‎ ‎‎‏‏‎ ‎•‏‏‎ ‎‎‏‏‎ ‎Sarcasm not correctly flagged with a /s.

Am I banned from Steam?
No - not yet. But you should refrain from making posts like this in the future. Otherwise I will be forced to issue an additional -rep, which may put your posting privileges in jeopardy.

I don't believe my profile deserved a -rep. Can you un-rep it?
Sure, mistakes happen. But only in exceedingly rare circumstances will I undo a -rep. If you would like to issue an appeal, shoot me a private message explaining what I got wrong. I tend to respond to PMs within several minutes. Do note, however, that over 99.9% of -rep appeals are rejected, and yours is likely no exception.

How can I prevent this from happening in the future?
Accept the -rep and move on. But learn from this mistake: your behavior will not be tolerated on Steam. I will continue to issue -rep until you improve your conduct. Remember: Steam is a privilege, not a right.