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:

MIZD9 Men's 2 Baseball Players Turf Trainer Mizuno Shoes Black Red alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Players Baseball Red Shoes Mizuno Men's Turf Trainer MIZD9 Black 2 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Hiking Annex Low Shoe Black Merrell TRAK Men's qZ5WpcxcwI

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

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