Basically just reduced and caramelized applesauce. It's used as a spread for toast or can be used in foods as an ingredient.

Never heard of it myself either.

"Apple butter is the richer, prettier, and perhaps more sophisticated sister to plain-Jane applesauce. It’s truly the embodiment of fall, with colorful crisp apples cooked down and caramelized in spices, brown sugar and vanilla. It tastes great on just about anything, but we love it spread over an English muffin or warm scone (heavenly!)."

https://sugarspunrun.com/apple-butter/

https://littlespoonfarm.com/homemade-apple-butter-recipe/

Looks great 👍