An open license is like a special rulebook that says it's okay for anybody to use something that somebody else made or owns. It's kind of like sharing your toys with your friends.
If somebody creates something like a song, a picture or a book, they usually own it and nobody is allowed to use it without their permission. But if they put an open license on it, they're saying that it's okay for anyone to use their creation for certain things. Like, they might say that people can use it to learn from and build on, or to make money from, but they might not be able to use it to make new things that are just like it.
So, it's a way of sharing what you made with others, but still being able to keep some control and say how it can be used. Anyone who uses something with an open license just needs to follow the rules the creator set out in the license.