Sorry I am a beginner, I see this everywhere, please explain in laymens terms what an "esc desync" means?

a common cause for ESC desync is a motor stopping or jittering in the air when flying - when motor is not moving, ESC can not measure how its moving. It will then initialise a default "motor startup" procedure which takes its time and sometimes will work so you experience a short break out of your quad or it will not work and the quad tumbles to the ground

Even more simple explanation: Anytime a kwuad randomly falls out of the sky is a Desync