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:

Warm ECCO Warm Jab Grey Women's Toggle Grey Sandal Metallic alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Sandal Metallic Toggle Jab ECCO Grey Warm Women's Warm Grey 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]House Fun Shoe Brown Cute Fuzzy Warm Women Bunny Slippers Slippers Cozy Animal Hamburger xzx6Zvw

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