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:

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

ary = [] of RecArray
ary.push [Chunky Sandal Heel Bamboo Ankle Glitter Single with Blue Strap Band Women's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]White EU37 BABOUCHE 6" US sheepskin STELLA M Morocco Women's Dear q5O5YwR

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