A fuzzy guard is precise blocking based on how fast an attack is. Without a fuzzy guard, any unreactable mid and low are a mixup. However, if for example your opponent uses an i13 mid to mix up a 20 frame low without delays, you might block mid for 13 to 18 frames and then low parry afterwards, covering both. This is fuzzy guard.
Since most powerful lows are slow, an expert fuzzy guard is hard to break with sloppy offense. The attacker has to incorporate delays or vary their moves, & often choose between pressure or 50/50s. Many strings are also fallible to fuzzy guard.
Only knowledge and practice limit this technique, meaning an expert player could sidestep for an i13 DF1, duck for an i16 high homing move, stand up, and then low parry for an i23 low. Though by the time they'd done all this, the whiffed DF1 would have recovered.