Impressions

Studio logs an impression when your thumbnail was on a YouTube surface and on screen long enough to have been seen, so home, search results, suggested and the subscriptions feed all count. Plenty of places your video technically appeared are excluded, among them external sites, embedded players, notifications and anything inside the player itself.

Read it as reach. Impressions climbing while views stay flat usually means YouTube offered the video to more people and they went elsewhere, which is a packaging question. Impressions draining away from something that had been doing well usually means the system found another video to push.