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:

Nice André Denim Flat Women's Assous Sandal alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Nice Flat Assous Denim André Women's Sandal 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Sneaker Walker Stability Black Grey Medicare Hcpcs Diabetic A5500 Code Shoe Propét Men's qwCzEBA

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