Man I was all set to troll Toan or something when I saw this post in the post feed until I saw it was a new forumer.
Stagger is when you deliberately pause and delay your attacks, to give yourself some time to see if the other person is going to try and do something. So if you are fighting a Ragna, and he loves to Inferno Divider, instead of just trying to attack him as fast as you can and giving him the chance to Inferno Divider you, you will instead poke him once, pause and see how he reacts, then poke again, and so on. It tends to mess with the defender's reflexes, and sometimes might cause them to try and attack only to get hit.
Sometimes that tiny, slight delay will convince them that you are not going to do anything, and they will think they can successfully attack. But since you only delayed it a little, they will not be able to make it in time and you'll still hit them. Basically it's almost a trap to make them stop blocking so they get hit.
It's almost its own type of mixup.
It can also have the effect of actually making them block MORE. Why? Well, if they already tried to poke out a few times while you were doing this delay pressure, then they will be afraid the next time that they'll be walking into this trap. So they will hesitate next time there is a pause, and that gives you more freedom to try and do slower things without getting hit.