Solid ground, midstage, not immediately under threat of being pushed down or sideways into a lethal blast zone, thus losing a stock. At low damage near the beginnings of matches, combat takes place mostly over land. As damage piles up and skirmishes stray to one side or another, that changes. Brawlers punted off the land to either side have to try and recover back to safety, with limited tools to do so.
If they use up whatever midair jumps, movement-oriented special moves, and air dodges that might be useful to them and they haven’t made it back, they drop. But if they can make it back to the stage somehow, either touching down with feet firmly or hanging from a ledge, they restock all those options, useful for dropping off on purpose, or if you get immediately launched back out (it happens).
While midair movement options are restocked by either putting boots on the ground or hanging from a ledge, full ledge-intangibility properties aren’t restocked until you touch down with both feet, standing. Ledge-hanging and repeatedly tethering rapidly diminish in evasiveness, and trying to ledge-hang more than six times (or tether more than three) simply fails until standing again.