When a guy says keep it tight, he means that you should keep your body in good shape. This can be interpreted a few different ways. 1) He could simply mean that you should exercise and eat healthy so that you look your best. 2) He could be more specifically referring to sexual activity, meaning that you should stay tight down there so that sex is more pleasurable for both of you. 3) Or, he could be warning you not to tell anyone about whatever private thing he just told you - in other words, to keep it on the down low. No matter what his intention was, if a guy tells you to keep it tight, take it as a compliment!