Swordmaster's explanation is mostly correct, there are stages to focus depending on how long you charge it. However, the longest charge can withstand any single attack. You can test this in training mode yourself -- I myself did just now to confirm it. You can make CPU ganon Fsmash or neutral B (convenient, seeing as how those were the moves discussed) and time it so the attack hits right before Ryu/Ken's counterattack. If a ganon forward smashes in a match it is unlikely you can react fast enough to focus long enough to tank it, since, like Swordmaster said, the earlier stages of focus are breakable.
As you keep this in mind, remember some attacks may look like one but are actually two. Charizard's flare blitz is the prime example: you may think the top level of focus can beat it, but in reality no level can, because there is really a small hitbox that combos into the big meaty hitbox of the move.