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:

6 Charcoal Comp Men's Charcoal Work Baltimore Waterproof Magnum Boot Toe 0 alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Work Waterproof Baltimore Toe Magnum Men's Boot Comp Charcoal 0 6 Charcoal 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]VEE3NVY Black Port Shoes Vans Authentic Unisex Red Royale 1O6Sqpw

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