This champion's location is hidden from your enemy.
A champion that is stealthed can move around without the opponent being able to see it or target it directly. However, the champion can still take damage from cones and other abilities which don't target a specific champion. In addition, spells like Retribution and abilities like Manic which affect the closest champion usually still affect stealthed champions if that champion is the closest.
The common counter is Detection, and it's the one that is recommended to have in every bg, but it can be replaced sometimes by sources of global damage as damage destealths champions, bombs and area damage spells are often used also, however, abilities like combat awareness and and murk can protect champions from be revealed by damage. If the stealthed champion doesn't suffer damage, as in the case of loss of life or due to total damage reduction, it won't lose the stealthed condition