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:

Shoe Armour Under Gemini Men's Fire Blaze Speedform Phoenix Gray 3 Glacier Running Orange alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Men's Gray Shoe Blaze Speedform 3 Armour Under Gemini Orange Running Phoenix Glacier Fire 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Slide Under Breaker Ignite Sandal V 002 Black Men's Armour Graphite XwIqArX

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