What does Frostbite do?

What frostbite does is it fills up a bar. Once that bar is full, it does significantly damage to health, and stamina regen will decrease

It works similar to bleed, but instead of doing an insane amount of HP damage, it splits between HP and Stamina damage, reduces your stamina regen and slows your movement. Vordt has an AoE Frostbite 'aura' and the smaller Irithyll knights you find through the game also deal frostbite damage with the weapons you receive by defeating them. It's also good to note that like bleed, some bosses have a higher resistance (and those like Vordt) or complete resistance to Frostbite.