Steampunk is a pseudo-Victorian aesthetic, an alternate universe if you like, where certain technological advances occurred in the 19th century instead of later in the 20th.   Noir is referring to film noir, a style of filmmaking that derived from melodramatic detective and mystery novels that found popularity around the time of the Great Depression.  So steampunk noir is just combining the two, taking a Depression-era setting in a post-Steampunk world.