Coitus interruptus is a method of contraception in which, during sexual intercourse, the man removes his penis from the woman's vagina just before his orgasm. That way, the ejaculation of semen is not in the vagina but elsewhere.
Used by itself, this method is very unreliable, because pre-ejaculate may also contain (small amounts of) spermatozoa (sperm). Pregnancy is also possible if semen contacts the female vulva. Additionally, not everyone is able to note the arrival of orgasm in a timely manner, and interrupting just at climax can be psychologically difficult, as it goes entirely against instincts and reflexes all designed to encourage procreation by encouraging ejaculation to occur deeply within the vagina.
The advantage of coitus interruptus is that it can be used by people who have religious objections against other methods of contraception, and that it requires no artificial devices. The disadvantage, as stated, is that it is extremely unreliable.