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:

Skate Shoe Supra Black Black Black Toe Leather Men Ineto Round alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Ineto Supra Black Toe Round Black Shoe Men Leather Black Skate 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]W and SIKEBIKE Smartodoors All Cycling 001 MTB Men's SD01 II Shoes Silver Women's SD black Road MTB and wESfI

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