A trout in a feeding lane sees thousands of natural insects float past every hour. They drift the way the current carries them — no swimming, no twitching — just food-shaped debris moving at the speed of the water around it.
A dead-drift is a cast that mimics that. Your fly travels downstream at exactly the speed of the surface current with no pull from your line.
The thing that breaks a dead-drift is drag. Your fly line, sitting on faster or slower water than your fly, tugs the fly off its natural path. The fly speeds up, slows down, or — worst — leaves a tiny V-wake behind it. To a trout, that wake reads as "not food."
You fix drag by mending: lifting the line off the water and re-placing it upstream of the fly. A clean mend buys you another two or three feet of clean drift before drag returns.
Most fly-fishing days come down to one question: how long can you keep your fly drifting like nothing's attached to it.