These are useful to look up a specific property on another type. The syntax is similar to an array index in vanilla Javascript.
type Person = { age: number; name: string; alive: boolean }; type Age = Person["age"]; // type Age = number
The indexing type is itself a type, so it's possible to use unions, other types, keyof, etc.
type I1 = Person["age" | "name"]; // type I1 = string | number type I2 = Person[keyof Person]; // type I2 = string | number | boolean type AliveOrName = "alive" | "name"; type I3 = Person[AliveOrName]; // type I3 = string | boolean
[[20210331123636-mapped-types]]
https://www.typescriptlang.org/docs/handbook/2/indexed-access-types.html