With alias you can give a type a different name:

alias PInt32 = Pointer(Int32)

ptr = PInt32.malloc(1) # : Pointer(Int32)

Every time you use an alias the compiler replaces it with the type it refers to.

Aliases are useful to avoid writing long type names, but also to be able to talk about recursive types:

Women's Sandal Flat Denim André Nice Assous alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Flat André Denim Women's Nice Assous Sandal 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Stone Shoe Skate Scissor Supra Olive qEXICxw

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)
end