What is the difference between serializable and non-serializable data?

I was reading Redux documentation, and this is what I read:

It is highly recommended that you only put plain serializable objects, arrays, and primitives into your store. It's technically possible to insert non-serializable items into the store, but doing so can break the ability to persist and rehydrate the contents of a store, as well as interfere with time-travel debugging.

I want to know when can an object be called serializable, and when not?