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:

Red Men's Wedding Gaorui Business Shoes Wine Shoes UPS Lace alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Lace Shoes Wedding Shoes Business UPS Wine Red Men's Gaorui 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Tactical Men's 7 Multicam Black Boot Recondo Bates Inch xTRwqIAA

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