RRH stands for Remote Radio Head. It is a special component sitting on top of cell tower that mainly performs following procedures.
i) Convert optical signal to electrical signal
ii) Convert electrical signal to optical signal
iii) Up/Down conversion
iv) Amplification of RF signal
The most important feature of RRH is the item i) and ii) meaning that eNB/BaseStation can send and receive data directly to the top of the tower via optical fiber which has almost negligible energy loss comparing to the conventional method that use thick RF coax cable.
Why we use RRH ?
As I briefly mentioned above, the biggest motivation is to reduce the energy loss while transfering the signal back and forth between the antenna (top of the tower) and processing unit at the bottom of the tower.
In conventional base station, they usually put Amplifier at the bottom and transfer the signal via thick Coax cable. But the pathloss of the coax cable is relatively large, they had to amplify a lot more to compensate energy loss by Coax cable.
To reduce this problem, an idea of putting Amplifer unit at the top of tower (this unit is called TMA(Tower Mounted Amplifier)), but I don't think this idea were adopted widely.
Then RRH was introduced and I think it has been so successful.