Roughly speaking a legal document stating what the author lets people do with their open source code. There are multiple types of license.

The core element is basically saying “I have released this code as opened source and anyone can use, no one else is allowed to claim it’s theirs and own it.”

Depending on the type of license there are often additional caveats. Like saying anyone who releases updated or altered versions have to release them under the same license or that people using it can not sell it.