You become hidden. As you do this, you may roll 1d20 + SNK. You may choose to use this as your Hidden DC.
When you are hidden, you cannot be seen. This condition is always associated with some hidden DC it takes to see you. Unless otherwise noted, it's your passive sneak (10 + SNK). Other creatures will only be able to see you (or know that you're there) if they use some relevant ability that lets them do so, if their passive perception (10 + PER) is greater than your hidden DC, or if enabled to by some other effect. In most cases, a creature trying to see you will have some check which must pass your hidden DC.
While hidden, if someone knows where you are but is unable to see you, any attacks they make against you are done with disadvantage. Similarly, any attacks you make against someone that can't see you are done with advantage. Attacking causes you to no longer be hidden.
While hidden, you must not Move faster than your hidden speed. Any other movement effects require a DC 15 SNK feat save to remain hidden.