To overstate something is to talk about it in a way that makes it seem more important, more serious, or bigger than it really is. You overstate a problem, a success, a danger, or a number. It is close to "exaggerate," though overstate often sounds a bit more careful and shows up in writing. A set phrase is "it is hard to overstate how important this is" — meaning it really is that important, so you are saying you are not going too far. The opposite is understate: make it sound smaller or less serious than it is. Though actually, people use overstate more for claims and numbers than for wild stories. If someone invents a whole tale, we more often say they exaggerated, or they lied. You overstate a thing; you do not usually "overstate to someone" the way you tell someone.