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:

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

ary = [] of RecArray
ary.push [Platform Heel Blue Fashion Sandals Slippers Beach High Hotmarzz Women's Flip Wedge Flops Summer Stylish 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]High Top Fashion Sneaker ECCO Soft Black Men's 8 zCtxWwIvq

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