Overstacking refers to the practice of using equipment to raise your character's resistance to well above the 75% maximum, in preparation to duel players who will use -resistance equipment or spells. It's helpful against Aura Enchanted monsters with the Conviction aura.
For instance, 150% cold resistance is usually useless, but becomes essential when dueling a character with the Lower Resistance curses, or the Conviction Aura, or a Sorceress with Cold Mastery.
Overstacking is also useful before venturing into Uber Tristram as part of the Pandemonium Event, since Uber Mephisto wears a very high level Conviction aura.