A licence is an official document which gives you permission to do, use, or own something.

If you say that something gives someone licence or a licence to act in a particular way, you mean that it gives them an excuse to behave in an irresponsible or excessive way.

If you describe a commercial activity as a licence to print money, you mean that it allows people to gain a lot of money with little effort or responsibility.

If someone does something under licence, they do it by special permission from a government or other authority.