The Sea of Fragments (カケラの海 Kakera no Umi) is a vast, otherworldly space containing countless Fragments (カケラ kakera), prisms representing the crystallization of human worlds and memories. Many different Fragments exist for every possible event and circumstance a world can experience; the Sea of Fragments thus represents a sort of multiverse.
Fragments are often likened to books and given titles to set them apart from one another. Most humans are unable to perceive Fragments or interact with them, let alone visit the Sea of Fragments, but there are exceptions.
Minagoroshi first establishes the Sea of Fragments as a setting. Rika Furude is able to travel between Fragments and access the Sea in-between time loops, thanks to the powers given by Hanyuu. It is also where Rika's alternate self Frederica Bernkastel resides in.
The Sea of Fragments is first officially named in End of the Golden Witch. It is comparable to the Meta-World, being a higher dimension that only beings like Voyager witches and above can access and traverse. Bernkastel and Lambdadelta are two such Voyagers, spending all of their time traveling the Sea in search of Fragments to entertain themselves with.
Character bios in Twilight of the Golden Witch show that the characters Willard H. Wright, who isn't a witch but used to work under the Great Court of Heaven, and Lion Ushiromiya are also able to traverse the Sea of Fragments.