Basically, if the move hits fast enough, and is an actual hit (Like say, Joe Higashi's Screw upper DM) then a Guard point can absorb the entire move, doing no chip damage to you, and let you get the advantage with an attack, after the move ends.
If a move is kinda slow (Like, say, maybe King's double strike), the Guard point will just absorb 1 blow, and you'll continue your move animation... and get hit by the second blow.
Basically, certain frames of moves have Autoguard / Guard points. If soemthing hits those points, during those frames, and KEEPS hitting fast enough, the character will stay in that frame, until the attacks are over.
An important note VS similiarity to sentinel and hulk in MvC3: they have a limited number of hits in their Autoguard (theirs is more like super armor). So if you lands those hits before their animation happens, they LOSE their guard, and enter normal hitstun. That doesn't happen in KoF, they can take unlimited hits, as long as the hits come fast enough.
Note: You can beat autoguard by combing from the guarded move, into a command throw, or a move with quick invincibility. This is pretty fun to do!
Moves that have these properties? I might miss a few:
K' EX Minute Spike
Takuma's EX Kyoukugen Ko Ou
Maxima's Vapor Cannon, Double Vapor Cannon DMs, Ground CD, Far C, Close C, Crouching C, Far & Close D, & EX M19 Blitz Cannon
