battery calibration

[happy_face]

I've got a Dell laptop which is a couple of years old. recently when I've switched it on, it comes up with a big message saying the battery needs recalibrating and do I want to do it. I did it once and it took almost 3 hrs.
Next time I switched it on the same message came up asking if I wanted to recalibrate.

Not sure what this recalibrating means, can anyone help? Is it a fact of getting a new battery?

thanks