Academic tenure is like a more secure job for professors at a college or university. Imagine your teacher at school having a magic spell protecting them from getting fired. That's what tenure is like.
When a professor gets tenure, it means they have been working for the school for a certain number of years (usually around 5-7 years) and have shown that they are really good at what they do. This includes being a good teacher, doing research, and helping the school in other ways.
Having tenure means that the school can't just fire a professor for no reason or because they disagree with what the professor says or thinks. Before a tenured professor can be fired, the school has to show that the professor did something really bad, like breaking the law, being completely incompetent or harmful to students, or not doing their job.
Tenure is important because it helps professors feel more secure in their job and it lets them do their work without worrying about being fired because they expressed unpopular opinions or ideas. This way, they can focus on teaching and doing research to help students learn and make important discoveries.