Serializable means that the data can be converted to pure text without losing information. Usually, when talking about serialization in JS, people mean that one can do the following

// data contains your data, for example:
// const data = { foo: 'bar', };

const data2 = JSON.parse(JSON.stringify(data));

with data and data2 containing the very same data after the above program executed. What that means is that you have to be careful with certain structures. For example, you cannot serialize a cyclic reference, because a naive text representation can neither store references, nor cyclic behavior.

// data contains your data, for example:
// const data = { foo: 'bar', };
// data.self = data; // <-- cyclic reference

// The following will fail with `TypeError: cyclic object value`
const data2 = JSON.parse(JSON.stringify(data));

So, if you are not sure if your object is serializable, use the above code snippet and see if everything works out.

Just for understand : Serializable means that the data can be stored. And ,as @maruru said, the data can be converted to a string, then deserializable without losing information. You can define your own serializable Object by defining the toString() method. In my case, i use static method parse and stringify (like JSON)

class A{
    ...
    toString(): string
    static parse(serialized:string): A // deserialize - convert to instance
    static stringify(instance:A): string // serialize - convert to string
}

In the meanwhile, serialize mean exactly : convert into a format that can be stored ( in a redux store by example) ( not always string, buffer is good too, or binary or anything that can be stored ;) )