So the first part is mostly just lingo, IIRC correctly it comes from fretting two strings at once, ie. stopping them. It started on classical strings such as violin, so some of this shit is lost in the mists of time.
But to the bigger point, double stops are used as all sorts of intervals, 3rds, 4ths, 5ths, you name it. They can harmonize the chord you are playing over, or imply a chord, etc... You know the notes, and triad basics I believe, so I would suggest thinking about how these notes stack on top. An easy exercise on the E and B strings is to go up the A minor pentatonic scale with double stops, so C and E (first fret B string, open E string) D and G (third fret B and E string) etc., all the way up the neck. The first group is a major 3rd, then second a perfect fourth. You can listen to the different sounds they make and get ideas where to use those textures and can hear how each sounds relative to each other. Stick on an A drone track and noodle away.
As far as right on wrong, it's really does it sound good or not, that's it. I typically play double stops on the G and B strings. Hendrix played a lot on the A and D strings. There's no wrong answer.