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:

Men Yoga Traveling Jogging Hiking Fishing Blue4 Shoes Breathable Oumanke Sneakers Running Sports alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Traveling Yoga Sports Hiking Sneakers Men Jogging Fishing Oumanke Breathable Running Blue4 Shoes 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]2 Three Men's Quarter Strike Football Alpha NIKE ORANGE Cleat BLACK TEAM atTFw

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