Woven textiles in general are created on a loom through interlacing / interweaving yarns or fibers, which run at a 90 degree angle to each other.
The warp or warp yarns are the yarns that run longitudinal in the fabric, or one can also say down the roll of fabric.
The weft or weft yarns are the yarns, which run widthways in a fabric or horizonally on a roll of fabric.
Once fabric has been cut from the roll, it can be quite hard to tell which yarns are the warp and which are the weft yarns.
One way of telling which is which is that warp yarns are quite often finer / thinner than the weft yarns.
Below image shows a close-up of Rivoli colour Prussian Blue.
The dark blue yarns are the warp and the gold coloured yarns are the weft.