Alcohol that you store in a convenient location at the side of a road or out of public sight. This is done usually when you are told that you cannot bring your own alcohol onto a premises, and so you have to stash your remaining alcohol somewhere.
The intention is that when you leave the bar / club, you have a nice pick-me-up to keep you going well into the early hours.
The location can be marked with a stone, or the container itself contained under a bunch of stones.
Let's be honest... no-one is going to abandon good alcohol!
Guy 1: Shit! We're supposed to be at the bar, but I still have a fuck-load of alcohol to drink.
Guy 2: Fear not my friend! Grab some of them rocks over there and we'll plant some Road Booze for later!