Forum warnings are given for breaking the Forum Rules, for example, for spamming, insulting and advertising. After a number of warnings the member becomes "watched" and his activities on forum are tracked, he also might get muted upon receiving more warnings while having this status. The "watched" status may be removed automatically after some time or for beeing active and helpful on forum. You can check your current warning level on your profile.
Once warned you will receive a message stating the reason you were warned for.