Sadaqah Jariyah means Continuous Alms/Charity. A continuous charity is an action that someone does that remains active even after the person is dead. For example, if a person digs a well then lots of people can use it and for a very long time - even after the person dies. It is a very effective type of charity that serves better than just giving money as the person who initiated the action will get thawab (plus points) even after he dies.
A popular slogan that explains the concept has been used by different charities in their advert. It goes something like this: "Give a man a fish and he can eat for a day, but give the man a boat and a net to catch fish with then he can feed himself and his family for a life-time."