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:

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

ary = [] of RecArray
ary.push [RN NIKE Red Black Free Shoe tough Blue Men's Hydrogen Running 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Running 66 No Flyknit 2 Red Town Shoes Sneakers Men's Lightweight wFA7xqAngC

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