What is the 3rd option in an enum property for? If I look here I see the following:

https://www.blender.org/api/blender_python_api_2_66_4/bpy.props.html#get-set-example

# Enum property.
# Note: the getter/setter callback must use integer identifiers!
test_items = [
    ("RED", "Red", "", 1),
    ("GREEN", "Red", "", 2),
    ("BLUE", "Red", "", 3),
    ("YELLOW", "Red", "", 4),
    ]


What is the "" field for?
From the documentation you linked:


  items (sequence of string tuples or a function) – sequence of enum items formatted: [(identifier, name, description, icon, number),
  ...] where the identifier is used for python access and other values
  are used for the interface.


The third item is therefore the description, which is used for the interface.