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:

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ary = [] of RecArray
ary.push [Buckle Enzo Romeo Size with Print Fashion Crocodile on Elastic Slip Gray Runs Shoes Big Men's Dress Santcro Loafers Half 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Shoe Work 09H Ferrara Webs Step Spring Blue Women's UqIHWxZ

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)