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:

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

ary = [] of RecArray
ary.push [Slip Indoor with On Mineral Men's Cooling Outdoor Comfort Devin Slipper for with Foam Memory Microsuede 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Graphite Men's Etnies Marana Shoe Skate Vulc x6wHAq7

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