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:

Short blue Boots Boots Sneaker Outdoor Fur RUN Winter Snow L Men's 6811 alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Boots Snow L Winter Boots Short Outdoor RUN Men's Sneaker blue Fur 6811 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Phluff Solid O'Neill Daddy Men's Black 5xwYa4YqzZ

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