A citizens band or CB Radio is a short range 2 way radio used to communicate with other cb radios. They're useful for communication between multiple vehicles traveling off-road together. In fact, a lot of organized off-road events and guided trail rides actually list them as a required item. Especially when headed out to remote locations, cell phone signals are often very weak or nonexistent. A cb radio can become the only way to quickly communicate trail dangers to a vehicle behind you, or notify a trail leader of a problem.
There are hand-held radios that do not require to be permanently installed in your Jeep, which can be very convenient and easy to use, however they often do not have as great a transmission range as a permanently installed radio with a larger external antenna. In fact, even an expensive permanently mounted radio, will not perform well without a good quality antenna.