In a weaving, the weft is the set of threads that runs horizontally. The weft is moved through the threads of the warp to create the piece of cloth.
Weaving is accomplished by lacing together two sets of threads. These threads are put at right angles to one another. There are vertical threads (the warp) and horizontal threads (the weft).
Traditionally, weaving was done on looms. Some parts of the loom would hold the threads of the warp. They would allow the warp threads to be moved up and down. The threads of the weft would be on a shuttle. The shuttle would be moved back and forth. This would make the threads of the weft go over and under alternate threads of the warp. This process would create a piece of cloth where the horizontal and vertical threads would be interlaced forming a solid piece of cloth.
In weaving, the weft is the threads that goes horizontally.