A sin is a bad act. Particularly, it is a bad act against another person(s), an animal, a spirit, or a god.
In Abrahamic religions, such as Christianity and Judaism, it is an act that goes against what God wants. It turns people away from God and true happiness. In Exodus chapter 20, Moses was given the Ten Commandments by God, explaining important sins like murder and idolatry. This is not believed by all people, and not all societies have the same idea of sin.
There is a difference between sins and prohibitions. For example, Muslims and Jews say that it is wrong to eat pork, and Hindus do not eat beef. That is for them, but for others it is not wrong to eat these things.
Sin Facts for Kids. Kiddle Encyclopedia.