Gobo filter -> spot light. Image -> slide_map.
The gobo filter can only be used with spot lights. In theatrical stage lighting, a gobo (or "cookie," slang for Cucoloris) is a thin sheet of metal with holes used to break up the light beam into an irregular, more natural pattern such as tree leaves, etc. In the computer graphics world, gobos are sometimes referred to as "slide projectors" (or sometimes "projector lights," but note that in Arnold the gobo is not a light source itself, but a filter applied to a spot light source). Any texture map or procedural shader can be projected through the light.
The blending equation by which the gobo's slide map is combined with the lights output. 'Blend' creates a default gobo effect. 'Replace' multiplies the slide map over the spot light. 'Add' multiplies the spot light over the gobo. 'Sub' inverts the RGB values of the slide map. 'Mix' averages the results equally between the spotlight and the slide map.
The texture map used to create the gobo effect.