What does it mean to stack?

[Hysterior]

Sounds like Aurum Vale.

Bosses give a poisening debuff that "stacks" meaning that each time the numbers goes up, you get more damage.
But there are not enough fruits (that cures the debuff) to always eat them.
So you need to wait for your debuff to be at 2 (first boss) or 3 (last boss) before eating them.

[Iscah]

So you need to wait for your debuff to be at 2 (first boss) or 3 (last boss) before eating them.

I think the "three stacks for the final boss" is a bit more fluid now - probably that's safe for DPS but tank and healer are better to clear it after two stacks - there seem to be more than enough fruits at the current clear rate, but that may depend on your data centre. At current I'm regularly seeing the boss die around the point it casts Sow for the second time.

At any rate, waiting for three stacks as healer has led to a few deaths or close calls for me, so I'd rather not risk it.