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:

Magnum Charcoal Comp Men's 0 Charcoal Waterproof 6 Toe Boot Baltimore Work alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Boot Baltimore Comp Magnum Toe Charcoal Charcoal Men's 0 6 Work Waterproof 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]XIV Lebron Basketball Mens Anthracite NIKE Anthracite Shoes AFWHwOqn

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