What does it mean when the little see through hole in the battery is red instead of green?

 

The red that you see is a little plastic ball. There is also a green one in the hole. They are used to measure the specific gravity of the acid in your battery. It works much like the coolant tester used to measure the quality of your coolant. Red is not good. You might consider taking it to your local Ford dealer for a free battery test. Ford SMT