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:

Men's Perforated Chestnut Slipper Clog Dearfoams Microfiber Suede alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Slipper Chestnut Dearfoams Suede Perforated Clog Microfiber Men's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Women Chitobae Flops Lemonade Pink for Sandal Flip Irq5AwI

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