A marriage bar was a rule that said that women who got married were not allowed to keep their jobs. It was kind of like a big "no, you can't" sign that was put up in front of women who wanted to work, but also wanted to get married.
You see, a long time ago, people thought that women should mostly stay at home and take care of the kids and the house. They didn't think that women could do a good job working outside of the home. So, when women got married, many companies thought that they wouldn't be able to give their jobs the attention they deserved because they were too busy taking care of their new family.
This rule made it really hard for women to have a career and a family. If they got married, they had to give up their job. And if they wanted to work, they couldn't get married. It was like they had to choose between being a wife or being a worker.
Thankfully, this rule doesn't exist anymore. Women can get married and work at the same time. And lots of companies have realized that women are just as good at their jobs as men, and they shouldn't be punished for wanting to have a family too.