This is the thread that runs the length of the band. These threads run through the cards and are secured to either end of your loom (or yourself and stationary object with backstrap weaving)
The warp threads are easier to work with if they are not very a very stretchy, or fluffy material. Fluffy threads can get roughed up by the friction of running through the cards. Stretchy threads make it difficult to keep your tension steady.
Read more about materials here
The weft is the thread that runs back and forth along the width of the band. This thread is passed back and forth between the warp threads. It usually is not seen on the finished piece, except for the very edges of the band, or through the weave in a band using the missing-hole technique.
If possible the weft should be as thin as the warp thread, but better still a little thinner.
S & Z Threading