A Horcrux is an object in which a person has concealed part of their soul. When the soul is split and part is hidden in an object outside the body, that person is unable to die, as part of their soul remains earthbound and undamaged.
Lord Voldemort hides one of his Horcruxes in a stone basin, which stands on an island in the middle of a lake, concealed within a cave. The Horcrux is protected by the Emerald Potion, which fills the basin. The Horcrux in the basin is Salazar Slytherin's locket.