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:

Glacier Fire Orange Gemini Armour 3 Shoe Men's Blaze Speedform Under Running Gray Phoenix alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Blaze Armour Phoenix Shoe 3 Orange Gray Fire Gemini Under Speedform Glacier Men's Running 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Diamond Silver White Trainer Air Black Men's NIKE Eq1Tgw7

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