What are "mapped types" in TypeScript?

An existing codebase has this interface declaration:

type TForValue = boolean | string; interface ISomeInterface { parameters: { [parameter: string]: TForValue }; foo: number; }

What is the type or meaning of parameters in this interface?

How would I create an object that satisfies this interface? What syntax would I use for that?

My initial thought was that parameters is simply a map from string to TForValue, but that idea appears to be completely misguided - I cannot assign a Map<string, TForValue> to parameters nor construct a Map<string, TForValue> object from parameters, so my thinking is that parameters cannot be a map.

I have read the docs on "mapped types" but it was not helpful to me.

I have entered this syntax in TypeScript Playground and in my IDE (PyCharm), but neither shows any helpful completion for members of the parameters field.