If you are sanctioned, it means someone has put a "hit" on you.

When you are sanctioned, you can be attacked even if you're below 27 Health, and you can keep being attacked until you "die".

You can also be attacked by ANYONE and you don't get any experience from defending against someone while you're sanctioned.

When you "die" you are instantly revived with 1 Health.

Sanctioning someone else means that you'll pay anyone that can "kill" the person you want killed