What's "fit in" mean?

Native speaker’s answer
Rebecca
To "fit [in/into]" is an expression used when there is enough room or space for someone or something. You can use this to mean that one is able to wear a shirt, for example, or in a physical space. Ex: I think I gained weight, I can't fit into this shirt anymore. Ex: This apartment is too small. It won't fit two people and three dogs.