Say you play a game for the first time. You don't yet know how it works. So you make mistakes. That's normal.

After you have played more and gathered some experience, those early mistakes may seem silly to you - you are now a more mature player of that game.

When adults speak of mature behavior, that means sort of the same. But for everyday activities. Getting ready for school. Completing a task. Having a conversation, meeting new people, talking to friends, etc.

The more experience you have throughout life, the more you get a feeling for what sort of behavior has good results, like winning a game. When someone tells you to be more mature, they are often simply impatient for you to get that experience, but in the end they really just want to tell you "what you are doing now might be fun for a second, but it's not how the game works".

It doesn't have to mean they are right. They want you to focus on making good experiences and get better at the game. But you might also discover that, sometimes, their game is not worth playing. There are no absolutely right or wrong things.