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:

Blue Glitter with Chunky Sandal Single Heel Bamboo Women's Ankle Strap Band alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [with Ankle Sandal Bamboo Heel Chunky Glitter Blue Band Single Women's Strap 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Craze White Women's Cross TR Blue Silver Trainer Shoe Gel ASICS Diva 4 O7xZqwEOP

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