Torque is a roational force about an axis. In the case of disc golf, off axis torque is a roational force about an axis that doesn't go through the center of the disc in a direction perpendicular to the flight plate at the time of release.
OAT can cause the disc to act either more or less overstable than the disc would if the throw was clean. Most of the OAT you hear talked about in a negative manor stems from people trying to throw discs that are faster than the thrower can control. To get the discs to fly straight and far the thrower will learn to add OAT to get the disc to act more understable. If they were to throw a slower disc like that it would most likely flip and turn into a cut roller. The result you get are players who can only either throw really fast discs sorta far (350' max) but with little control and they are unable to get slower fairway drivers, mids and putters to do anything but flip.
The two biggest motions that contribute to OAT are wrist roll (either under or over) and lack of plane preservation with your arm and shoulders on your follow through. Rolling your wrist over (your palm is facing the sky immediately after the hit) and following through on a lower angle than the discs' angle at the hit cause discs to act more understable. The opposite cause discs to act more overstable. The former is a much more common problem.
OAT, when intentional, can also be used to control a discs's flight and shape lines. Say you want to have the disc flip from hyzer to flat, glide a bit, turn over, glide farther and then fade. You can throw a disc like a beat DX Eagle-X on a hyzer, but finish on an angle less than the hyzer angle. Controlling how much of a difference there is between the angles will allow you to control when the Eagle goes from straight to tracking right. Depending on how beat the Eagle is, the disc will either continue tracking right, fade back to straight and glide a bit, or fade back left. Once you get good at these types of shots (sometimes called roll curves), the negative effect of choosing the wrong beatness of fairway driver will have a small impact (perhaps 10') rather than a large one (30'-40' off for choosing the wrong mold).
Roll under can be used effectively too. Say you have a shot that's kind of short for a midrange, but there's a stiff left turn at the end. You can throw your mid with less hyzer than normal and roll under (or follow through on a higher angle) and the disc will either hold a hyzer the whole time or fade out earlier/harder than it would on a normal shot.
One of the easist ways to get roll under out of your throw is to practice throwing your most understable plastic on pure hyzers. Start off short and lengthen how far you throw, avoiding letting the discs flip up from a hyzer at all. Eventually you'll get the feel for how to control the wrist roll. Your slower discs will fly farther and be much easier to control and the speed you gain (or stop losing from OAT) might make the best longer range disc for you more obvious. You'll start seeing lines you never knew existed, using discs for shots you never imagined would work (I threw a flippy Gazelle ~350' into a headwind a couple weeks ago) and you'll amaze people because it starts looking like your discs are remote controlled.
