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

ary = [] of RecArray
ary.push [Sandal Strap Single Chunky Blue Ankle Glitter Band Bamboo with Heel Women's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Diadora Unisex Unisex Elite B White Diadora B Elite 4EFwxqIE

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