It is a (currently) .hrl-based repository of commonly used typespec "idioms" that are not available from OTP.
Just include typespecs.hrl in your code and you can start using its typespecs.
If you are using rebar, you can declare typespec as a dependency in rebar.config:
{deps, [{typespecs, "0.1", {git, "git://github.com/spawngrid/typespecs.git","HEAD"}}]}.
-type gen_server_state() :: #state{}
-spec handle_cast(any(), gen_server_state()) -> gen_server_handle_cast_result().