Customs are like the rules that people follow without even thinking about it. For example, when you go to a restaurant, you know you're supposed to tip your waiter or waitress. That's a custom.
In law, customs are the rules that people in a specific community have been following for a long time. These rules might not be written down anywhere, but everyone knows them and follows them.
For example, let's say you have a farm on the edge of a town. For years, you've always let people cut through your property to get to the town's main road. Over time, this becomes a custom. People expect to be allowed to cut through your property, and they get upset if you suddenly stop letting them.
In some cases, customs can be recognized as actual laws. For this to happen, the custom has to be widely accepted and consistent over a long period of time. When a custom is recognized as a law, it means that people who break it can be punished by the legal system.
So, in short, custom in law refers to unwritten rules that a community has been following for a long time. If the custom is widely accepted and consistent, it can be recognized as a law and enforced by legal authorities.