First Wort Hopping (FWH) adds a hop charge to the kettle as wort is still running in from the lauter, before the boil has technically begun — the hops sit steeping in progressively hotter, not-yet-boiling wort for the time it takes to finish collecting runnings, then continue through the entire subsequent boil on top of that.
A longer, gentler extraction than a standard 60-minute addition
Because FWH hops are exposed to heat earlier and for longer than a hop added right as the boil starts, some brewers and a handful of small studies report a smoother, more rounded perceived bitterness from FWH compared to an identical hop addition made at the top of a standard 60-minute boil — though the measured IBU contribution itself, calculated the normal way from total effective boil time, ends up similar either way.
A traditional technique that's genuinely easy to add to any recipe
FWH doesn't require any special equipment or process beyond timing a hop addition to go into the kettle as the lauter runoff begins rather than waiting for the boil to start — a low-effort way to experiment with whether the reported smoother bitterness character actually shows up in your own system and recipe, without changing anything else about a standard hop schedule.