cell phone amplifiers

Does anyone know what cell phone amplifiers are? I don't. I would guess that they are some kind of cell phone attachment to improve your cell phone signal. I'm not sure why people would need them - it seems like you can get good cell phone signals just about everywhere! Even in Africa there were lots and lots of cell phones and cell towers. One of my roommates in Africa had a cell phone, and she even found a signal when our ship was literally at sea - something like 10 miles from the shore!