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:

Roe Boot Fashion Grey Crevo Men's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Grey Fashion Boot Roe Crevo Men's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Tobacco Flop Sanuk Beer Men's Tan Cozy Flip nwx46RHqpA

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