The valve stem on your wheel lets you put air in and take air out of your tires. Most of use don't really pay much attention to them, until they stop working like they should, and that can be in any number of different ways. With most late model Jeep's, the valve stems include a TPMS or Tire Pressure Monitor Sensor which sends a singal to your Jeep letting you know what your tire pressures are and if any tires are low on pressure. The valve stems use a battery to power the TPMS and eventually those batteries will die. They're not replaceable, so you'll need to buy a whole new TPMS valve stem to replace them.
When you buy new wheels and tires, you'll also need to purchase a set of valve stems to go with them. Depending on your Jeep, you may need ones with TPMS or you may not. There are low profile valve stems that lessen the chance you'll accidentally rip out a valve stem on a rock or other obstacle while off-road. Valve stems are also available in different colors, like black or chrome, so you can match them to your Jeep.