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

ary = [] of RecArray
ary.push [André Sandal Assous Nice Women's Flat Denim 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Summer Flip Blue Shoes Comfort Beach on Cork Glitter Flops Sandals Sole Women Slippers Slip vEqx674

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