Modern batteries like a laptop battery have built in management systems which are intended to achieve the best performance and extend the life of the battery. However these systems can become confused about the battery's actual capacity for charge if it is frequently topped up and never allowed to almost completely discharge every so often. The battery develops a sort of false memory about its actual capacity to hold a charge. Calibration aims to restore the management system's measurement of the charging capacity by the user allowing the battery to get down to almost no charge and then recharging it back up to full capacity every so often. I think the recommendation is to do this every 3 months or so, depending on how often you use the laptop. It is important that you don't completely discharge the battery, however, as that could mean it will never recharge again.
Rather like making yourself a nice gin and tonic, and as the drink goes down topping it up with just tonic. The drink gets progressively weaker and unable to deliver the desired result!
