Basically what the title says. Zombies (and possibly survivors, too - I don't raid, so I wouldn't know) are invulnerable to both projectiles and melee for the duration of the stagger animation, resulting in weapons with high aps/kb dealing significantly lower DPS than is displayed on the weapon card during missions. It's most evident with the new Containment Arc-Burns, but the same behavior can be seen on any weapon with an aps lower than the duration of the stagger animation and a decent kb value, such as shotguns. I'm not sure if this is a bug or a feature, but making it so that zombies still take damage while being staggered or making the survivors stop shooting zeds during their invincibility frames would be nice.
I'm 95% sure that it's not just an issue with damage numbers not showing up, seeing how the arc-burns take much longer to kill a zed than an underleveled SMG with significantly lower displayed DPS, but if anyone has proof to prove me wrong I'd love to see it.