Not very! About 22 out of 100 women who use withdrawal as their only form of birth control for a year will get pregnant. Withdrawal is when a man takes his penis out of a woman's vagina ("pulls out") before he ejaculates or "comes" (has an orgasm). This lowers the chance of sperm from going to the egg. "Pulling out" can be hard for a man to do. It takes a lot of self-control.
Even if you use withdrawal, sperm can be released before the man pulls out. When a man's penis first becomes erect, some fluid may be on the tip of the penis. This fluid has sperm in it, so you could still get pregnant. Withdrawal also does not protect you from STIs, including HIV.