Free association is when people come together voluntarily to work and live together in a way that they see fit.
Marxism is when people believe that instead of having some people be really rich and others be really poor, we should all work together and share things equally.
Anarchism is when people believe that we shouldn't have any governments or leaders, and that we should all be free to make our own decisions and work together as equals.
So, when Marxists and Anarchists talk about free association, they mean that people should be free to work and live together without being forced to by a government or a boss. Instead, they believe that people should be able to choose who they work with and how they work together in a way that's fair for everyone involved.
In this way, free association can help people come together to create a more equal and just society, where everyone is able to have a say in the decisions that affect their lives.