Tire pressure refers to the air pressure (PSI, or pounds per square inch) inside each of your tires. Too much and too little are both bad. Much like other areas of life, it’s all a matter of balance.
To little air pressure could cause poor handling, poor fuel economy, reduced load capacity, longer stopping distances, and even a blowout! If you really let your tire get low or if you drive on it when it’s flat, you will end up with a substance like this in your tire; I call it “tire sand.” At this point, the tire is unsafe to drive on and must be replaced.
Too much air pressure can give you a harsher ride, poor/erratic handling, increased tire wear (particularly in the middle of the tire), and more road noise. Link back to this article to get the low down on tires and tire pressure.
If you don’t already have one (or 10!) you need to invest in a tire gauge. You can get a couple of inexpensive pencil-type gauges to keep in each car and a quality, accurate dial or digital gauge to make sure your el-cheapos don’t give you a terribly false reading. Keep your tires properly inflated for safety and economy!