In Six Ages, your Clan Magic is a valuable resource. You can spend it at Sacred Time, or keep it in reserve to deal with events.
Your clan magic is renewed at Sacred Time, although the exact amount you get varies—both by factors that you can control (clans with all-male or all-female circles get less magic, and those that represent all possible gods get more, for example) and factors that you cannot (Raven's whims and the state of the Gods' War). Also at Sacred Time, you can decide to allocate magic to various areas, like War or Diplomacy, to help your clan in that area for the rest of the year.
You can spend magic during the year in response to various events--for example, a taxing spirit rite will usually cost you magic. Other random events will cost you magic to begin with. Keep in mind that you need your magic to be above zero in order to perform a ritual.
There are fewer opportunities to gain magic during the year, but it is possible. For example, the Ceremony Venture will usually generate two points of magic, and you can also ask a clan that owes you a favor to pay you in magic. Explorers will sometimes find magical crystals, and some choices during events can grant magic as well.