Japanese people use 'W' to represent laughter. 'W' or 'WWW' stands for 'warai', which means laughter in Japanese. You could see it as a synonym for 'LOL' or 'Haha' in English. So when someone finds something funny, the Japanese say 'WWWW'. The more W's, the longer the laughter, similar to 'Hahahahaha'. This is mainly used in informal contact and less common in a professional context.