What does resting meat actually do?
It lets the temperature even out and the muscle fibres relax, so that less juice runs out when you cut it. The common explanation — that juices "redistribute" or get "reabsorbed" — is a simplification of something more mechanical.
During cooking, heat causes muscle proteins to contract and squeeze moisture out of the fibres into the spaces between them. The outside of the meat is far hotter than the centre, and that hot exterior is under the most tension. Cut into it immediately and that pressurised liquid escapes onto the board.
Resting does two things. The temperature equalises as heat continues moving inward from the hot exterior — a process called carryover cooking, which can raise the centre by several degrees, so meat should come off the heat below its target. And as the meat cools slightly, the proteins relax and the muscle's ability to hold moisture partially recovers. Thickening of the juices as they cool also helps.
The result is measurable. A rested steak loses noticeably less liquid to the board than one cut straight away, and the difference is visible.
Roughly: a steak or chop wants 5 to 10 minutes; a chicken 15 to 20; a large roast 20 to 30. Loosely tent it with foil rather than wrapping it tightly, which traps steam and softens any crust you have built.
It will not go cold. A large joint loses only a few degrees in twenty minutes, and serving on warmed plates covers the rest.
Two exceptions: very thin cuts have little to redistribute, and anything with a crisp coating is best eaten straight away, since resting softens it.