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:

Glitter Heel Sandal Blue Strap Women's Ankle Bamboo Band Chunky Single with alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Glitter Ankle Blue with Strap Band Women's Bamboo Single Heel Sandal Chunky 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Slippers Girls Paw Fuzzy Rainbow Moccasin Plush Rainbow Trimfit Xwd7qX

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