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:

Summer Fashion Wedge Sandals Blue Heel Beach Women's Flip Slippers Platform Flops High Hotmarzz Stylish alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Blue Hotmarzz Flops High Stylish Wedge Sandals Platform Summer Flip Women's Fashion Heel Beach Slippers 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]B US Star Basketball Ox D All Chuck Red Men M Shoe Women 8 Converse M 6 US Taylor Unisex wCqH6H

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

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