Bidirectional RF Amplifier is an amplifier that works in both directions. It has an power amplifier to increase transmit power and it has a Low Noise Amplifier(LNA) to increase receiver sensitivity. Due this LNA Meshtastic device can receive signals that it cannot receive otherwise.
There is a word of caution. Always check SMA patch cord to see if it fits the setup you are building. I have faced a situation where amplifier was not working due to provided cable. Just this provided cable was not for my setup. It started working by replacing the patch cord with correct/matching SMA connectors with my setup.
Meshtastic 2.4GHz Worldwide Band 4 Watt Bidirectional RF Amplifier Overview by Technology Master.