It's a tire knocker, sold in just about every truck stop in the US, Canada and Mexico. They run about 15 bucks.
When you do your pre-trip you walk around the rig and thump every tire. (there are at least 19 tires you check at least twice daily) You listen for the sound of the tire when it is hit. A tire with full pressure almost rings when you hit it, one that is low will make a lower note and one that is flat just makes a thud. Do it a few times and you can tell without a gauge. Heat won't tell you much on a big rig tire. They ALL get hot when loaded.