I was impressed by how well-dressed everyone was at the wedding.

well-dressed 是由副词 well 和过去分词 dressed 组成的复合形容词。well 在这里表示“很好地、得体地”，dressed 来自动词 dress（穿衣、打扮）。well-dressed 用来形容一个人穿着讲究、衣着得体，通常指在正式或重要场合穿着合适的服装。