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:

Grey Metallic Sandal Toggle ECCO Women's Grey Warm Warm Jab alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Grey Warm Grey Jab Women's ECCO Metallic Warm Sandal Toggle 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Men's Sneaker Max ONEMIX Walking Running Air Sports Shoes White Casual g8xvS

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