What is Metadata?

Can I ask what is metadata exactly? I haven't found anywhere any good description, only methods JsDoc.

According to Metadata Proposal you offer to add to any JS Object unreachable property [[Metadata]]. But anyone could read and write to it with specialized methods? Like 'defineMetadata', 'getOwnMetadata'? So it is more like getter/setter from OOP without direct access to property.

So I can make something like this to copy the behavior?

//structure will look like: 
const obj = {};

obj['Metadata'] = {
  customProperty1: {
    property: 'someValue'
  }
}
// Methods would look like this: 
Object.prototype.defineMetadata = function(metadataKey, metadataValue, target, key) {
  this['metadata'][metadataKey] = {key: metadataValue};
}

The issue names the Metadata Proposal and methods such as defineMetadata and getOwnMetadata. Start by comparing the proposal with the methods' JsDoc and the example in the issue. Done means documenting what metadata is, how it is accessed, and whether the proposed object-property model is accurate.