Multiple attacks in rapid succession can stagger characters.
Specific Attacks are more likely to stagger, and certain enemies are hard to stagger, experiment to learn how to avoid it and how to exploit it against your enemies!
Successful actions in combat such as dealing damage and perfect dodging will increase your Focus meter.
Inaction will eventually drain it.
Once the focus meter is full (blue bar under the health bar) you are able to activate Surge. This will rapidly drain your focus meter in exchange for a significant increase in damage, stagger impact damage and resistances.