A Clan Altar is a special Clan building that produces Valor and Experience point bonus for the whole Clan. To increase and maintain production of the bonus, Clans need to send Forest Essence to the Clan Altar.
Forest Essence can be obtained by completing personal goals in repeating events. You can read more about that here:
To upgrade the Clan Altar, your Clan will need Magic Seeds, which you can get from the Gifts of the Forest event.
The maximum bonus granted by the Clan Altar is 100%.