YouTube Shorts are a format on YouTube, showing short content. Originally under one minute in length, the current time limit of a short is 3 minutes. These numbers are liable to change and might not be current at the time of reading. Shorts are also shown in a "portrait" orientation, with about twice as much height as width.
YouTube Shorts are basically a way to respond to the popularity of TikTok (and Instagram Reels), showing short videos for short attention spans. Unlike normal YouTube, where videos are shown by search or browsing and selected, YouTube Shorts play automatically, based on, of course, some crazy algorithm hidden away somewhere. That leads to something like doomscrolling, only most YouTube Shorts are not very doomy, instead focusing on short explainers, personal stories, graphics, cute pets or animals, and often reused content from movies, musics or interviews. Viewers can idly scroll for hours, watching content that goes in one eye and out the other. The term "brainrot" might come to mind, although it isn't quite as bad as that.
Creators are also paid minimally for Shorts, assuming they are monetized in the first place. A short with a million views might earn the creator around $50, as opposed to the $5,000 dollars a creator would earn for a million views. Even though many creators are reusing material, $50 per million views is rather small. (NB: These numbers are estimates, YouTube doesn't make this information publicly available). The big draw for most creators making YouTube Shorts is to get more subscribers and views on their longform videos.
YouTube isn't the best, or the worst service, but it is emblematic of what the internet is like in 2025.