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 Round Skate Men Shoe Leather Black Black Supra Toe Ineto alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Ineto Black Leather Black Shoe Round Toe Skate Black Supra Men 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Lugz Smooth Sneaker Cargo Fashion Men's Black HwrHqBx

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