Masturbation involves touching or rubbing the parts of the body that create pleasurable sexual feelings. It's one of the ways that we get to find out what feels good for us sexually.
For most guys and girls, their first pleasurable sexual experiences involve masturbation. Surveys report that by age 18 almost 100% of guys and over 50% of girls have tried masturbating. Many young people discover masturbation all on their own, while others learn about it by talking to friends. Not everyone is okay with masturbation and it is still considered "dirty" by some religions and by some parents. But, there is nothing wrong with masturbation and the choice is yours.
For guys, masturbation almost always involves stroking the penis until it gets hard and then continuing to rub it to create pleasurable feeling or to have an orgasm (also called "cumming") which includes an intense physical feeling that usually ends with semen spurting out of the penis.
Girls use many more ways of pleasuring themselves and this can involve touching or stroking the clitoris, the breasts and any body part that creates feelings of sexual pleasure. Masturbation doesn't always have to include an orgasm. It can be simply a way of feeling pleasure with one's body.
There are many myths about masturbation and its effects. One is that it causes blindness or grows hair on your hands. This is absolutely untrue.
If you are masturbating by yourself, you cannot get an STI. However, you might get an STI if you decide to masturbate with a partner, or share sex toys with someone else.