A symbol drawn next to a character's head to represent laughter, mirth, happiness or chatting noise. Looks a bit like ╀─╀, but with a curved line. Commonly used for laughter or the noise of a crowd that happens in the background, or in a scene that is otherwise silent.
Not to be confused with the Chinese radical "艹", which is often used as a shorter and more euphemistic form of "cao" (Chinese for "fuck") when typing/writing.