What does it mean to stack?

[MrThinker]

I thought I knew what "stacking" was when fighting bosses in a group. But I'm confused some of the terminology (some I have forgotten but will have to ask again later here).
People say "stack 3", or "eat 2" & not understanding what it is and why.

So there is 'stacking' in which a group of players pile in on each other to deal with a mechanic. And then there are 'stacks' for buff and debuffs. The first is self-explanatory. The 2nd is the explanation provided by Hysterior, except it can also apply to beneficial status effects.