Stagger mechanic

Can someone explain to me what exactly ‘stagger’ means? Is it when the enemy’s poise meter is depleted, or when an attack causes them to flinch? I feel like the new mechanic nerfed focus builds. There’s a way to regenerate focus through staggering enemies (enchantment stats), but I don’t fully understand how stagger works. Moreover, does the lightning effect make enemies stagger more quickly?