Some enemy attacks can break Ryu guard while he's blocking. When his guard is broken, you can cancel the broken guard status into a special dodge, which have more i-frames than the normal one. This is a pretty well known mechanic amongst NG players of all levels. Guard glitch, instead, is a rather obscure technique which is not explained in the game tutorials, and is rumored to actually be a glitch. When you get guard broken, release the guard button and then hold it again, Ryu will still remain in the broken guard animation, but his guard is actually reset, so he can block again. The guard can be reset as many time the player want, he just has to release and then hold the block button any time his guard gets broken.
I tested this technique in NG2, NGS2 and NG3RE. I've heard that it can be executed in all NG games included the very first one, though as I said I've only heard about it and never tested it in the other games in the series.
Guard glitch is really useful for learning patterns from bosses or medium sized enemies, or simply to guard against attacks without moving, if needed, giving you more tactical freedom. You can choose if you want to reset your guard or do the special dodge according to the context. This technique is also used to perform the Elizabeth loop.
Very personal, useless note: I prefere calling this technique "guard reset".