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:

Sneakers Jersey Authentic True Unisex Low Adults' Smoke Slim Top White Vans alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Adults' Top White Slim Vans Authentic Unisex Low Sneakers True Jersey Smoke 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Composite Safety Leather Crosstrex Low Thorogood Men's Toe Black SqtgUwF

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