Stacking is collecting precious metal as a physical product, rather than having it as part of an investment portfolio where it's digi ownership of a portion of a giant bar, often just bullion & mostly gold or silver.
Some people stack platinum & a few other metals.
They are not usually bars/coins purchased purely for their numismatic value, but they can be.
I do both.
A stacker is someone stacking metals which are often in coin form & can be purchased individually or by the tube/box of tubes although you can buy in bar form too.
It's one of the few hobbies I've had where I would make a profit if I gave it up tomorrow & sold my stack (that's usually only true over the long term unless you get lucky with your timing).
Flippers try to buy & 'flip' (sell) precious metals quickly to make a profit.
Stackers stack theirs to keep, sometimes forever (to leave to their kids), sometimes just until some decided point in the future, e.g. to supplement retirement, to meet a massive unexpected expense (new car/surgery) or even as a house deposit/paid in full depending on how big their stack is.
If you have too much though, you really need a safe deposit box elsewhere to store it in, so you only get to mess with your whole stack from time to time.