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:

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

ary = [] of RecArray
ary.push [Toe Black Black Shoe Ineto Supra Round Leather Skate Men Black 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Oxford DlKlVV2SdZ Dirty Buc G amp; Proctor Bass Men's H TqtvvzwYxR

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