The creature knows you’re present, somewhere, but doesn’t know where you are. They can use the Search action to try to find you. They can use abilities with the Area tag to affect you, if they can guess where you are, but cannot target you with abilities that require line of sight or being able to see or detect you. If they somehow guess your location and use a non-Area ability targeting that square, they must Succeed on a DC 10 Flat Check or the ability automatically Critically Misses and has no effect on you.
While you are Undetected by a creature, they are Flat-Footed against your attacks, and your attack rolls against them have Advantage.
You stop being Undetected when you do something that would alert the creature to your position or reveal yourself to their vision or other Precise Sense.