In-market audiences are Google's segments of people actively researching or intending to buy in a category, based on recent search and browsing behavior. They are the closest Display and video get to search-style intent.
Intent, on non-search surfaces
On Search you reach intent through keywords; on Display, YouTube, and Demand Gen you approximate it with in-market audiences, people whose behavior signals they are shopping the category now. They are stronger than affinity or demographic targeting for driving action.
Where they fit
Use them to seed an audience signal, to layer onto Display and Demand Gen, or as an observation layer to learn which segments convert. They are inferred, so treat them as directional and let conversion data confirm which ones actually pay off.