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

ary = [] of RecArray
ary.push [6 Baltimore Charcoal Waterproof Work Charcoal Comp 0 Men's Magnum Boot Toe 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Egronomic grey Sole Sandals 9 Waterproof Women 4 Size 5893 Comfort Signature 5 qf4pBHw

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