If a ship runs aground, it gets stuck because the water is too shallow — the bottom of the ship is sitting on sand, mud, or rock. The ship has not sunk; it just cannot move until the tide comes back, or until someone pulls it off. You almost only use this about boats and ships (and sometimes, jokingly, about other things that get stuck). It works as an adjective and as an adverb: the ship is aground; it ran aground. Sailors and news reports use it; everyday conversation uses it when a ship story is in the news. Though actually, people on land say a plane is grounded (kept from flying) and a child is grounded (not allowed out) — those share the word ground, but aground is the water word.