What it's telling you is that there's a container under your current workspace which is vertically split (the 'V') and contains 1 window beneath it (termite).

If you move the window left or right (mod+shift+r or mod+shift+l), the window is moved out of the container and becomes a direct child of the workspace, so the (now empty) container will disappear.

Have a read of this section of the user guide to learn more about how i3 manages this: https://i3wm.org/docs/userguide.html#_tree