To weave fabric on a loom, a thread (called the weft) is passed over and under a set of threads (called the warp). It is this interlacing of threads at right angles to each other that forms cloth. The particular order in which the weft passes over and under the warp threads determines the pattern that is woven into the fabric.
Before the Jacquard system, a weaver's assistant (known as a draw boy) had to sit atop a loom and manually raise and lower its warp threads to create patterned cloth. This was a slow and laborious process.
The key to the success of Jacquard's invention was its use of interchangeable cards, upon which small holes were punched, which held instructions for weaving a pattern. This innovation effectively took over the time-consuming job of the draw boy.
When fed into the Jacquard mechanism (fitted to the top of the loom), the cards controlled which warp threads should be raised to allow the weft thread to pass under them. With these punch cards, Jacquard looms could quickly reproduce any pattern a designer could think up, and replicate it again and again.