A Stealth check is the act of applying the hidden status to yourself against as many creatures as possible.

A creature who is hidden from another creature cannot be directly targeted by that other creature, as the other creature does not know where they are.

This is different than invisible, or in darkness, or obscured. Creatures under such effects cannot be seen, but signs of their presence are still notable: their footfalls, their breath, their odor, etc. A creature that cannot see you still knows where you are, and can still target you (though they typically take some penalties to their direct attacks against you).

But if you are unseen, you can make a Stealth check to become hidden. Doing so is typically part of your movement on your turn, and you are hidden from creatures whose Passive Perception you beat with your ability check. Typically, this is a Dexterity (Stealth) check, but it may also be an Intelligence (Nature) check, such as when attempting to camouflage yourself, or a Wisdom (Survival) check to lay an ambush.

A creature can take an action to actively look for you, typically using Intelligence (Investigation) or Wisdom (Perception). Taking an action to look lets them roll, which might give them a higher chance to notice you.

Being hidden from one creature doesn't mean you are hidden from others — you can be hidden from one orc, and completely obvious to the other orcs, for instance, if one orc is on the other side of complete cover.

Once you are hidden from a creature, you remain hidden from that creature until you end your turn somewhere where you are seen by the creature. For instance, if your invisibility spell ends while you are hidden from an orc, and you're standing right in front of the orc…you won't be hidden from them at the end of your turn.

However, during your turn, you can become hidden again. If, for instance, the invisibility ends at the start of your turn, but by the end of your turn you move to a totally obscured area, you can become hidden again by rolling again.