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:

Skechers Studio Meditation Women's Kicks Navy Bandana alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Studio Kicks Navy Women's Skechers Meditation Bandana 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]British Suede Slip Collection King Shoes School Old Black On Black 66Arq

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