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:

Nice Women's Flat André Assous Sandal Denim alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Sandal Nice André Women's Denim Flat Assous 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Industrial Composite Brown Wolverine Men's Boot 6'' Bandit Toe wtXt8p

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