The Effective Altruism movement originates from the realization that the ways to do good good are not always self-evident. This means that, very often, we don't have as much impact as we could have, or even that we do more harm than good. This article by 80,000 Hours explains this issue quite well, as well as giving a good introduction to the importance of EA! Also check out the introductory page on the official EA website.