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:

Comp Men's Work Charcoal 6 Toe Magnum Baltimore Charcoal Waterproof 0 Boot alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Boot Men's Baltimore Work Magnum 6 Charcoal 0 Charcoal Toe Comp Waterproof 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]State Slippers Ohio NCAA Buckeyes LICENSED Womens Happy College Feet Men's Sneaker and OFFICIALLY YqvPwzv

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)