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:

Fur Winter L Boots Snow Sneaker Boots Outdoor blue 6811 Men's RUN Short alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Boots 6811 Short Sneaker Fur L Men's Snow RUN blue Outdoor Winter Boots 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Genuine 2 Shoes Leather brown Washington Dress Men's Marc Oxfords Bruno WqHxtnv8OU

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