Three main definitions:
1.) To say something insulting or rude about another person, either to their face or behind their back.
2.) To make a statement that is exaggerated or flat-out untrue
3.) To say something negative behind someone's back that you wouldn't say to their face.
1.) She likes to talk a lot of shit when she gets drunk, "That guy is a fucking creepy stalker", when all the guy did was try to start up a conversation with her.
2.) "I can skate so ill, I did a 50/50 to a frontside boardside and landed doing a manual" or "My cousin's best friend in high school was Tony Hawk, and one day I met him and we chiefed some killer bud together!"...statement made my a poser "skater" who can barely do an ollie while in place on a skate.
3.) "She's such a whore", said in a serious way, by a girl who is actually good friends with another girl who just happens to be dating a guy she kind of liked; a comment made out of flat-out jealousy or envy.
All general examples of what it is to talk shit.
To go smoke some pot, while driving around. This is performed in situations where it isn't acceptable to smoke weed inside. This is applicable in situations where there is no important destination except to leave and smoke pot discreetly, as in not inside any building. Common: at home (to avoid family), around friends (who don't like that you smoke, or who want to mooch off your weed), or if you are working a job that isn't serious.
You're working somewhere (not that serious of job) and you have a co-worker friend, or a friend stops by, and they smoke the ganja. "Wanna go drive around?"
You live at home with your parents and you want to go smoke by yourself, or much preferably, with a friend. "I'm going to go drive around," you tell your parents, and it really is true. They just don't know you are going to hot-box the ride.
Any situation where smoking the herb must be done discreetly (as in most situations) so no other parties know. As long as you don't smell of it, hopefully; in most observed cases.