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:

Black Turf Trainer MIZD9 2 Players Shoes Men's Baseball Mizuno Red alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Mizuno 2 Men's Baseball Shoes Players Red Trainer Black Turf MIZD9 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Swiss Sandshell Sneaker Classic Spiced Men's K Ash 88 Coral TxaXdTH

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