Questioning the syntax of a TypeScript feature as outlined in the documentation

I have been studying the Typescript advanced types documentation, available at this link.

Within the documentation, there is an example provided:

interface Map<T> {
   [key: string]: T;
}

I grasp the concept of the type variable T in Map.

However, my confusion lies in the syntax used within the interface declaration, specifically in this line:

[key: string]: T;
  • Could someone explain in plain English what the above line signifies?

  • Additionally, can anyone point me towards resources explaining the usage of square brackets for keys in TypeScript interfaces?

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