It's simple, really. There's a file in the phone that keeps track of the relative battery usage of different apps and services. Some people think that by deleting that file, then charging and discharging the phone in various patterns will "calibrate" the phone somehow. According to the people who designed the battery circuitry for Android, it's an urban legend - that battery doesn't need to be calibrated. In fact, there's no "calibration" for the battery. The terminal voltage of a lithium battery is the approximatestate of charge, and that's what's being shown. (You can't determine the actual state of charge without destroying the battery - taking it apart and measuring certain things chemically.)
How to extend battery life? I assume you mean time between replacing batteries. Recharge when the battery is between 40%-60% discharged. NEVER let the battery overheat. If you leave it in a hot car in the summer, you're killing it. Never use it when it's very cold. (If you're outside in a Minnesota winter, keep the phone in a shirt pocket, or in a little case that hangs under your armpit.) If you need more time between charges than 60% of the battery will give you, if the phone has a replaceable battery, buy a spare and switch them when the one in the phone gets down to 40%. (Keep 2 spares if you use the phone so much that you need 150% of the battery's capacity.) Or buy one of those huge external battery packs. You can get them up to about 20,000mAh these days, and if you need more than that for one day's use, you need a longer extension cord.
But as far as "calibration" - that's a myth. Lithium batteries should be conditioned, but that's different. If the battery's been sitting on the shelf for more than 6 months (always use the battery down to about 40% charge for long-term storage), fully charge it, then use it until the phone tells you to charge it (the only time you should discharge it that far) and fully charge it again. Three full cycles of full charge, full discharge. You do this to new batteries as well - you have no idea how long they've been sitting on the shelf. Some people will tell you that I don't know what I'm talking about, but the batteries in my Motorola V551, made around 10 years ago, are still working fine. (They get swapped about every 6 months if the phone isn't being used, every month if it is.)
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