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:

DAWGS Sandals Pink Arch Z Support Womens Soft alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Support DAWGS Womens Sandals Z Arch Pink Soft 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]G7014 W10 Farm 8" Lacer Men's Boot Georgia Ranch SPR Boot Work qBaHfx

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