Many new paladin powers and some of the new feats subject a target to your divine sanction. Being subject to it means the target is marked by you for a duration specified in the description of the power or feat. Unless otherwise noted, the mark ends before the specified duration if someone else marks the target.
Until the mark ends, the target takes radiant damage equal to 3 + your Charisma modifier the first time each round it makes an attack that doesn’t include you as a target. The damage increases to 6 + your Charisma modifier at 11th level and 9 + your Charisma modifier at 21st level.
Divine sanction is meant to complement divine challenge. You can use divine challenge to mark one creature and use divine sanction to mark others. Divine sanction has fewer restrictions than divine challenge so that you can easily use the two in concert.
Published in Divine Power, page(s) 82.