A stack of things is a pile of them.

If you stack a number of things, you arrange them in neat piles.

If you say that someone has stacks of something, you mean that they have a lot of it.

If someone in authority stacks an organization or body, they fill it with their own supporters so that the decisions it makes will be the ones they want it to make.

If you say that the odds are stacked against someone, or that particular factors are stacked against them, you mean that they are unlikely to succeed in what they want to do because the conditions are not favourable.