Masturbation is a little bit like sex, but with yourself instead of another person. It's when you give yourself an orgasm, usually from touching the genitals.
Masturbation is not harmful, in fact, it is good for you. When women orgasm their bodies release hormones that make them feel good and reduce pain. Studies suggest that when males orgasm often in their teens and twenties, it may lower their chances of getting prostate cancer later in life.
There are no negative effects of masturbation provided you don't do anything too crazy, masturbate with sharp objects, or overdo it and make yourself a bit sore. All of that stuff is pretty easily avoidable, however. So feel free to masturbate. Most people do.