Watertight is a simpler term for Manifold.(Basically a 3 dimensional mesh which has a well defined surface area and a volume. ) Watertight is just the idea if you submerged it in water, the water would only contact 1 surface of any face of the model, not both sides of any face.
So with that reasoning, a plane object would have a surface area , but not a volume, so not watertight(non manifold). If the plane was extruded, forming a cube,it would have a volume now and a surface area, thus watertight.
As for our submerged object a well defined surface is when any face on a surface exposed to the water, its face normals should be pointing in the direction its surface is in contact with the water.
Sorry for this simplified explanation. The proper mathematical explanation gives me a headache. For a good little program that can fix several mesh errors, download netfabb basic. But nothing beats proper modelling techniques, including checking normals and manifold several times in its creation process.
Keith