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:

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ary = [] of RecArray
ary.push [Summer Heel Sandals Beach Platform Flip Hotmarzz Women's Slippers Fashion Stylish Flops Wedge Blue High 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Men's Noctu Tsugi puma Black PUMA Puma Black White Shinsei Sneaker Puma Bt6qxz5wqd

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

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