upside-down box radical (no. 13)
冂 is a radical character representing an upside-down box or a wide enclosure. Its standalone meaning is limited to this radical sense, and it is not used as an independent word.
冂 is a pictograph of an inverted box or a wide boundary. Its exact historical development is uncertain, but it likely originated as a simple enclosure shape.
Picture a box turned upside down, with its open side facing up. The two vertical strokes are the sides, and the top horizontal stroke is the bottom of the box.
For キョウ, imagine a box that is too きょう (today) to be used—it's an upside-down box that nobody wants. きょう -> キョウ.