In general, 3d solids are defined by their bounding surfaces. I.e., one defines one or more surfaces in space, which are considered as bounding surfaces of a body. However, this only makes sense if the surfaces meet at all points without gaps, so that everywhere inside and outside are clearly defined for the body. So there must not be a gap between 2 adjacent boundary lines either.
To express this figuratively, one speaks of a "watertight" set of surfaces.