Read Advanced Operators chapter of The Swift programming language.

By link jumped to quite old NSHipster post Object Subscripting.

Reading about @objc and @objc(name) jumped to Attributes chapter of The Swift programming language.

It’s interesting that page by link above links to Programming with Objective-C.

Read Subscripts chapter of The Swift programming language.

Subscripts are not limited to a single dimension, and you can define subscripts with multiple input parameters to suit your custom type’s needs.