What does it mean those who forget about their are doomed to repeat it.

I think you are missing here" the past":

Those who forget about the past are doomed to repeat it.

This means that we have to study the past, the actions and consequences of these actions, and learn from them, and if we are not doing this, we can make mistakes that could otherwise be avoided. (Example: ignoring ethnic and religious tensions has led to wars in the past, but these wars could have been foreseen).