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:

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

ary = [] of RecArray
ary.push [UPS Shoes Shoes Red Gaorui Wine Men's Wedding Lace Business 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]White Benassi NIKE Grey Wolf Print Summit Jdi Mens TwwqFB0U

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