You can use one repeater box, but most common is two boxes. Thieves first use a device that listens for the particular low frequency the vehicle sends out to check if the key is nearby, and the device then retransmits it at a higher frequency, such as 2.4Ghz or anything else that will easily travel much longer distances. The second device, held by the second thief, takes that high frequency signal and replays it again at the original low frequency. The keyfob sees this low frequency, and goes through the normal challenge response it would as if it was physically next to the car.
This happens back and forth a few times for the entire challenge/response between the key and the car, and the two devices are just relaying that communication over a longer distance.
Using these devices, the thieves create a bridge that stretches from the vehicle all the way to the key in the victim's pocket, home, or office, tricking each into thinking they're next to each other, allowing the door to open and the truck/car to start.