The principle "don't drink, don't smoke, don't fuck" which came to be in the late 80s by Ian MacKaye and the band Minor Threat.
Most straightedge kids nowadays flaunt the fact that they're straightedge by drawing X's on their hands, wearing clothing that says "Drug Free" or "XXX" or "Poison Free", because for some reason they feel the need to let everyone know they're drug free.
Some (but not all) straightedge kids are sXe
because they feel the need to fit in to a group, and they can do so by avoiding toxins.
Straightedge is not to be confused with hardline
kids, who are a violent version of straightedge and kick the asses of smokers and drug doers, because for some reason they believe people should not make their own choices.
Straightedge kid: I'VE GOT THE STRAIGHTEDGE! I'm a person just like you, but i've got better things to do than sit around and fuck my head; hang around with the living dead! Snort white shit up my nose, pass out at shows! I don't even think about speed that's just something I don't need! I've got the straightedge!
Kid 2: Um...calm down. I don't need your beliefs shoved down my throat, thanks
Straightedge Kid: I'VE GOT THE STRAIGHTEDGE!
Kid 2: I know. I know.
Straightedge Kid: NEVER WANT TO USE A CRUTCH! I'VE GOT THE STRAIGHTEDGE!
Kid 2: Great. I know.
Straightedge Kid: I'VE GOT THE---
Kid 2: SHUT THE FUCK UP. I GET IT.