Steamer is a system of projections developed by Jared Cross -- a high school science teacher in Brooklyn -- and two of his former students, Dash Davidson and Peter Rosenbloom. It is currently used by Fangraphs as its primary projection system for individual players.
According to Steamer's website, the projection system got its name because St. Ann's High School goes by the nickname the "Steamers." The system began simply as part of an independent research program at the high school, on which Davidson and Rosenbloom teamed up with Cross.
Like other projection systems, Steamer uses past performance and aging trends to develop a future projection for players. It also uses pitch-tracking data to help forecast pitchers. On Fangraphs, the projections are updated daily and predict each player's numbers over the course of the remainder of the season.
Obviously, no one is claiming that every one of Steamer's predictions will come true, but it is widely regarded as one of the most accurate predictors in the industry.