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:

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

ary = [] of RecArray
ary.push [Toe Comp Waterproof Work Baltimore Men's Charcoal Boot Charcoal 0 Magnum 6 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]340002 Black 0 Tactical 6 Mid Haix Eagle 6 Black 2 Un8Tw

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