ROTFL is internet slang. The letters stand for rolling on the floor laughing. You type it when something is so funny that — in a cartoon way — you would fall down laughing. Nobody is usually on the floor. It is a loud, extra way of writing “that is very funny.” It belongs to older internet chat, with LOL and ROFL (almost the same joke, a little shorter). Younger people now are more likely to send an emoji, lol, or a sticker. You almost never say ROTFL out loud as a real word; you write it. Though actually, if you use it in a formal email, it can look childish or dated. In Taiwan, people more often type 哈哈, 笑死, or just drop in 😂.