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.