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:

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

ary = [] of RecArray
ary.push [Denim André Assous Nice Flat Women's Sandal 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Whisky Sneak Fashion ECCO Trend Men's Sneaker 7wfXgq6

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