Receiver and Antenna diversity 101

Diversity technology for FPV and RC systems has become an intrinsic part of the hobby but what does diversity mean, what does it do and how is it achieved? Just as importantly, can passive receiver diversity really work on FPV goggles like the new low-priced Eachine EV100 units or will it create more problems than it solves?
Joshua Bardwell's review of the EV100s:
RCSchim's review of the EV100s