The medieval belief was that the universe is made up of concentric spheres with earth (aka land) at the middle, then water, air, the planets each in their own sphere, and then the firmament, which was an outer ring with holes in it that were the stars. To get around the fact that this implies that the earth should be entirely encircled by water, some of them theorized that the earth sphere actually floats to the "top" of the water sphere, so a small portion of it, the land we live on, would stick out. The rest of the earth sphere was contained within the larger water sphere.
Something like this, where you can see that very innermost circle is off-center.
Keep in mind that they did not have very accurate maps when they came up with this theory, nor did they know about gravity at all. This belief was based more in theology than any kind of reasoning, but until the early 15th century that was pretty much all anyone ever thought about. In the early days of the renaissance the Europeans rediscovered the ancient Greek philosophers' theories about the size and shape of the earth and started to think a little more critically, then the fall of Constantinople happened and they gained a new sense of urgency because their source of income was threatened.
Same as it ever was.
Edit: If you're interested, I learned all this from this book: https://smile.amazon.com/Fourth-Part-World-Earth-America-ebook/dp/B002SRL3FO