That t() is the type t() in the Range module, or written otherwise Range.t()

Any member of the type %Range{first: integer(), last: integer()) is also a member of t().

The t(first, last) means, that you also can create a range from other types, you could for example spec a function to accept Range.t(float(), float()).

The syntax of the typelanguage is part of elixir as described in the Typespecs section of the elixir manual.

Types and typespecs are used in two ways:

1. As documentation that humans know what kind of arguments a function can cope with. A reader of the spec disjoint?(Range.t(), Range.t()) :: boolean() knows that they need to pass in 2 ranges and not 2 integers or even an integer and a string. And also they know that this function will return a bool value, meaning either true or false.
2. Dialyzer can use this information to tell us if we pass correct types into the functions we use.