may be a bit too technical....

...but it's something they won't bother to tell you when you buy the vehicle, but there will come a time when you will need to know. And probably find out from someone who as already taken your tires off your car, and sorta holding them hostage, waiting for you to pull out your checkbook or credit card. Info that is most likely buried someplace in the Owner's Manual no one gets around to reading until it is 'oh, crap' time.

There is a little sensor on you tire, there where the stem is that you would add air.You have it, I have it, all God's chillun, who are driving vehicles manufactured after 2005 or '06 have it, according to the guy at the tire store. Uncle Sam said it into being, so it's got to be true.

This little device has a little battery in it to send a message to the computer that keeps your systems going (you do know they are all run with computer chips, now, right?And when the computer has a glitch you will be sitting along the edge of the road, crying?). When the tire pressure gets low, for any number of reasons: an actual leak, cold weather, road conditions, etc., the sensors in each tire communicate with the computer, to let it know there is a problem.

But... here's the sneaky part: the life of the battery is much shorter than the life of the sensor. And it is designed so that you cannot replace just the battery. You have to buy a new sensor to the tune of $80 plus, plus whatever they'd charge at the tire store to install it. So that little light on the dash that the computer can turn on to warn you about problems with your tire maybe just be telling you that the battery in the sensor is dying.

You can, could, possibly, maybe drive it another ten thousand miles with the light on, but you never know...There might be legitimate problem: like that little boy in the folk tale who cried 'wolf' so often, the townspeople ignored him as the hungry canines ate his flock. Then you will be sitting out there alongside the road, waiting for road service to come and put the donut on, since the guys at the tire store put the (now disabled) tire on with a power wrench, and you cannot get the lug-nuts off by hand. In the dark. In the rain...
Not that I would want to be the one to paint a picture of the Worst Case Scenario....