A Google carousel is a row of rich (i.e., visual) results that searchers can interact with. They're common on mobile devices, but you can see certain types on desktops as well.
Carousel are known to have a profoundly negative impact on Google's ability to crawl a site. Specifically, they: Can slow down site speed (depending on the type of slider used, and how many/what resolution images) Oftentime create multiple H1 tags automatically for each image loaded into the slider – very bad for SEO.