During a trial, a lawyer isn’t permitted to speak directly to the jury outside the opening and closing statements. But the opening statement is (theoretically) a preview of the evidence, and the closing statement is a summation of the evidence. So to actually present the evidence, lawyers use witness testimony. ”I saw the robber point his gun at the victim.” ”The blue car ran a red light and hit the plaintiff’s car.” And so on.
A lawyer also gets a chance to discredit the testimony of her opponent’s witness by asking things like “what kind of gun? How far away from the robber were you? Were you behind the victim, or behind the robber? How dirty were your glasses?” or “Were you watching the car, or the light? Where was the plaintiff’s car? My client’s car has a dinosaur sticker on the bumper: did you see any such sticker on the blue car?” Now we get to hearsay.
Hearsay is, briefly, testimony by proxy. The witness is not telling the jury what he saw: he’s telling the jury what someone else claims to have seen. This is problematic for a number of reasons.