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:

Sam Women's Ankle Dansko Grain Strap Clog Sand Full Dollar alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Dollar Sam Ankle Women's Strap Dansko Sand Grain Clog Full 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]On Loafer Brown Bush Strap Slip Nunn Men's Toe Monk Cap Newton zyx8vgqw

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