Fuzzy guard is a type of an option select where both standing and crouching guard are used to beat multiple enemy options. Sometimes it is used without a trigger(dinstance, move) and sometimes with. Sometimes it is required to be done with specific timings and sometimes it is done without specific timings and triggers.
It can be used in any situation where the timing of an opponent's mixups aren't quite the same.
Also refers to quick switching between standing block and crouching block or low parry in hopes that mix up will be blocked, without committing to one option, and giving it away
The simplest form(but not the only one) of fuzzy guard is with strings that can't be delayed. For example, Bryan's 1,2 has both a mid and a low extension, but the mid is faster, so the defender can do standing guard into crouching guard to block both options. In this case there's a specific trigger to start fuzzy guarding - Bryan's 1,2 string, and you need to duck with a very specific timing - after the 1 extension(if Bryan does 1,2,1) and before the 1,2,3 extension hits you(if you don't duck in time).
In neutral vs Paul at distance 1.5 where qcb3 and qcb4 will reach you but his hopkick will not. You can be doing backdash, d/f with the same or different timings(no specific timings required and moves are not performed with any specific timings) to fuzzy guard his qcb3 and qcb4. In this case there is no any specific triggers like distance or a specific move but it is still considered fuzzy guarding.
Another example of fuzzy guarding in neutral is to backdash, d/b in neutral at distance 1.0-2.0 against Kazuya to duck his electrics and block hellsweep at the same time as well as to block his f,f+3(Kazuya) because by the time he dashes in you won't be ducking anymore. In this case the trigger to start and stop fuzzy guarding lows is distance and the goal is to make electrics whiff, block their hellsweep and f,f+3 mid at the same time. In this case there is a trigger to start and stop fuzzy guarding but no specific timings to doing that.
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 might go unpunished.