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:

Assous Nice Women's Flat André Sandal Denim alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Flat Assous Denim Sandal André Women's Nice 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Primeknit Mens Tubular adidas Doom Sock zxtngqS

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