A TMB (Tower Mounted Booster) amplifies the radio signal to and from the base station. It is placed in the tower, just next to the antenna, which allows the booster to compensate for the power losses in the feeders and connectors.
The booster not only compensates for existing losses in the system, but more importantly increases the output power of the cell and thus the reach of the base station both outdoor and indoor.
The TMB contains both a booster for the base station transmitter (TX) and a signal amplifier for the uplink (from the mobile phone to the base station, RX) - a TMA. This combination provides optimal balance in coverage both for uplink and downlink.