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:

tough RN Blue Shoe Red Black Running Men's NIKE Hydrogen Free alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [tough Blue RN Free Men's Red Black Shoe NIKE Hydrogen Running 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]3 Armour Blaze Shoe Fire Running Gemini Under Speedform Orange Men's Phoenix Glacier Gray R6TwxqI

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