Auto-patrolling is an online single-player feature that allows you to interact with other players' characters as NPCs.
After you have played online with one of your characters for a while (single or multi), your character may appear in other players' online sessions as NPCs while you are offline. They can either appear in single lobbies to recruit in a team for battles and parallel quests, or appear in parallel quests as time patrollers in training.
The next time you log on to the server with that same character and other players have used/fought him/her, you will be briefed on what battles and parallel quests they have been in with/against other players and earn experience points, money and items from them.
You earn this achievement as soon as you log on with this character and they have participated in 7 patrols.