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:

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

ary = [] of RecArray
ary.push [Beach Blue Women's High Summer Fashion Wedge Sandals Heel Flip Stylish Platform Flops Slippers Hotmarzz 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Gray Dickies Mens Everyday Black Shoes wwU1qt

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