These terms pertain to the yarns used for weaving. The lengthwise yarn stretched tightly on the loom is called the warp thread and the yarn that's used for the interweaving is called weft thread. The warp yarn has to be really strong so it doesn't snap when subjected to high tension.
The images below taken from the book Weaving and Dyeing in Ancient Egypt illustrate the high tension the warp threads are subjected to during the weaving process. Click on the title to download the book from the Arizona Textile Library.
Why Should You Learn to Differentiate Between Weft and Warp?
While sewing garments, care should be taken to ensure that the weft thread of the fabric is parallel to the ground and not the warp thread. This is because the weft thread is weak and does not take well to wear and tear. A garment sewn with the warp parallel to the ground tends to tear more easily.