A reset is an action that resets the combo counter and thus the damage scaling (hence the name). Most resets are done in one of two ways:
-with a throw
-with an attack that hits in a direction opposite of what you’d expect
For example, in the corner with Magneto, you can do launch, roundhouse, airdash d/f, short, forward, and then kick throw when you land. You can then follow up the throw with low short, low roundhouse, Hyper Grav, whatever. Or if you think they’re going to tech the throw, do low short+Psylocke, low forward, whatever. Or you can let them tech it then guard break them (you’re in normal jump mode when you tech a throw). Or you can do the throw early (launch, roundhouse, airdash d/f, kick throw while in the air). As you can see, Magneto is best able to take advantage of resets, since he has so many ways with which he can attack at any given time and both of his throws can be comboed out of.