What does Filling mean?
The yarns that are the lengthwise, selvedge to selvedge horizontal and carried over and under the warp.
Rawr Denim explains Filling
Filling, or weft, threads are the horizontal threads that pass through the warp threads via the shuttle during the weaving process and are perpendicular to the selvage. Typically, filling threads are not as strong as the warp threads due to the fact that they incur far less stress and tension. In jeans, the filling threads are usually white or ecru and are only visible on the wrong side of the denim.
Here is the filling: