Exactly what “hidden” means in game is not completely obvious.
Hiding When you try to hide, make a Dexterity (Stealth) check. Until you are discovered or you stop hiding, that check’s total is contested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence.
You can’t hide from a creature that can see you, and if you make noise (such as shouting a warning or knocking over a vase), you give away your position. An invisible creature can’t be seen, so it can always try to hide. Signs of its passage might still be noticed, however, and it still has to stay quiet. (PHB p. 177)
So we know you can take an action do do it, and that when you are hidden from a creature they don’t know (exactly) where you are.
One further hint comes from the Unseen Attackers and Targets section:
If you are hidden–both unseen and unheard–when you make an attack, you give away your location when the attack hits or misses. (PHB p. 195)
Does “hidden” just mean unseen and unheard, like that section suggests? (also unsmelt, unfelt, untasted – generally unsensed) Or does it mean something else?
Some implications if this were the case:
There would be some extra niche cases that allowed easier access to being hidden.
creatures out of sight of a PC that had the Deafened condition would effectively be hidden from the PC
The Silence spell could be used to stop enemies without line of sight to a PC from knowing exactly where they/their party are in its radius (effectively “Mass Hide” as long as they have a line of sight blocker and stay in the radius)