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:

Size Half Loafers Romeo Enzo Fashion Elastic Gray Men's on Runs Buckle Print Crocodile Santcro Shoes Slip Big with Dress alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Fashion Loafers Size Shoes Dress Romeo Elastic Runs Santcro Crocodile Big Slip Gray Men's Buckle on Half with Enzo Print 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]for Leather Casual Milano Men's On Slip Men La Shoes Loafers Comfortable Classic Business Dress 17aqwfR

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