The "birth control shot" is injected every three months (or 12-13 weeks) in order to remain effective.
The shot injects a hormone called progestin, which can prevent ovulation, so therefore, sperms cannot fertilise an egg because there's no egg released from the ovaries.
However, this method of birth control does not prevent people from getting STDs, so, doctors also recommend couples to practice other barrier birth control methods like condoms in order to prevent themselves from getting STDs.