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:

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

ary = [] of RecArray
ary.push [Men's Armour Glacier Under Phoenix Gemini Orange Gray Blaze Shoe Fire Speedform Running 3 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Cognac Andrew Men's Loafer Casual Vegan Ahimsa x46nTqP

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