Receivers are your control link to the quad. They provide Telemetry data back to the transmitter so you can have audible callouts, warnings when your battery is low or your Link Quality is bad.
I recommend Crossfire for long range and maximum penetration, it's 900mhz is able to go over 55km in open air, which means alot of layers of concrete and rebar. The new NanoRX Pro even has 500mw broadcast power for longer range telemetry.
Tracer is the new low latency protocol using 2.4Ghz with faster packet rates and optional LoRa (alpha testing).