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:

Hendrickson Boot Men's Black Chukka Leather UGG alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Black UGG Hendrickson Boot Leather Chukka Men's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Fashion Canopy Reebok Dark Green Sage Sneaker Men's GW St Furylite p7XxwtqA

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