Just cause in employment law means that an employer has a good reason for firing or disciplining an employee. This is different from firing someone without just cause, which means the employer is letting the employee go for no good reason.
So, think of it like being in kindergarten. If you do something bad, like hit your friend, your teacher will discipline you by making you sit in time-out. The teacher has good reason to do this because you broke the rules and it's important for you to learn not to hit people. This is just like an employer disciplining or firing an employee who breaks the rules or does something wrong – they have good reason to do it to help teach the employee the right way to behave.
But, if the teacher just called you out of class and said you couldn't come back tomorrow without any reason, that wouldn't be fair. And if an employer fired or disciplined an employee without a valid reason, that wouldn't be fair either.
So, just cause is an important concept in employment law because it ensures that employees are treated fairly and not fired or disciplined without a good reason.