Laughing is an expression of enjoyment at something humorous. The letters 'X' and 'D' make up the basic emoticon symbol for laughing, 'XD'. You'd probably use it only for a joke that made you laugh so hard that you almost peed in your pants. It is sometimes written as "xD" or "XDDD". It seems to symbolize somene laughing loudly and hardly with their eyes closed.