Frass is a general term that means the things that insects and their larvae leave behind.
It contains excrement from all the things they consume as they so along like plant material, wood, human food, and other materials. Frass will look different depending on the insect type and what their food source is.
Frass can look like little bits of dust, rust, or sawdust, or whatever the insects have been consuming.
Frass also contains chitin, the main component found in the exoskeletons of insects and shellfish. The nutrients in frass are in a readily available form that allows it to function as efficiently as a mineral NPK fertilizer.