Safe Place is a feature that teleports a player to their last used healer when they have no usable Pokemon left. This ensures that a player has a usable battle ready Pokemon on them at all times and cannot avoid aggression battles.
No /back location is set when Safe Place is triggered, which means you lose any exploring progress you may have made. If you didn't have a last used healer set, it will teleport you to the Poke Center at spawn.
If you run out of usable Pokemon during battle you may also lose any pocket money you had on you, up to one third depending on your Pokedex progress, unless you have a Amulet Coin in your inventory, It will be consumed instead. Money stored in the bank is not effected by blacking out.