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:

Microsuede Mineral with with for Men's Memory Foam Cooling Slipper Devin Outdoor Comfort Indoor Slip On alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [with Memory Slip Cooling Slipper Outdoor Comfort with Devin Mineral for Microsuede Indoor Foam Men's On 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Soft Anti Flat House Bath Dark Casual amp;KATE 03 Womens Slip Khaki Sandals Slide Shower Slipper WILLIAM Couple Men PqfYzA

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