A collection of short stories that take place all throughout the Monogatari series, each featuring one of the girls discussing some sort of supernatural-seeming phenomenon with Koyomi.
Almost all of the episodes are effectively stand-alones that resemble the main series entry they take place near. A couple however (8 & 12) have explicit ties to the core storyline and won’t make much/any sense if you haven’t seen several of the main entries.
More Information:
aniDB
Wikipedia