It depends on the context but we usually use it like this : 가슴이 미어지다 / 사람이 미어 터지다. 미어지다 means squeez, aching, or even hurting. So when you say 가슴이 미어지다 it means heartbreaking feeling, or something very sad and painful. When you say 사람이 미어 터지다, that means there are so many people are squeezed in to one place. Imagine subway in rushhour... Hope it helps!