Talking about straight edge, well straight edge refers to a subculture of hardcore punk, In its simplest form, straight edge is a philosophy of staying clean and sober: meaning refraining from using alcohol, tobacco, and any other recreational drugs.The term was coined by the 1980's hardcore punk band Minor Threat in the song "Straight Edge".
The Symbol X
The letter X is the most known symbol of Straight Edge, and is worn as a marking on the back of both hands, though it can be displayed on other body parts as well. Some followers of Straight Edge have also incorporated the symbol into clothing. A variation involving a trio of X's (XXX) is often used in show flyers and tattoos. Also, it can be a pun based on the fact that three X's are a cartoon way to signify alcohol or poison.