The FC (Flightcontrol) is the heart of the copter. There the pilot's control commands are calculated with the values of the position sensors and passed on to the motors. The FC detects the position by built-in acceleration and rotation rate sensors, which are evaluated by the firmware.
All electronics are connected to the FC - the RC receiver, the ESCs / motors and, for FCs with built-in OSD (On-Screen-Display), also the camera and the video transmitter. An OSD also offers the possibility to display telemetry data - including battery voltage, flight time or connection quality - in real time in the camera image.
Some FCs also have an integrated PDB, which allows the power lines of the ESCs to be connected directly to the FCs.
FCs also differ in the firmware they use - the most popular ones are Kiss, Betaflight and Raceflight.