Warp and weft are the weaving or embroidery terms for the directionality of the threads that make up a loomed or woven fabric. Warp threads are the threads that run along the length of the yardage (up-and-down, vertically) and parallel to the selvage (horizontal axis). Weft threads are the threads that run from selvage to selvage (side-to-side, horizontally).
Any fabric created on a loom will have a warp and weft thread. This threading or weaving is how you turn thread or yarn into fabric. This includes evenweave, which means an equal number of warp and weft threads per inch, and plain weave, which is characteristically has a tightly woven warp and weft, and many other embroidered fabrics.
Nonwoven fabrics such as felt, vinyl, suede, and leather do not have warp and weft threads.
Why Is This Important?
Most of the time, when you embroider on fabric, you will not have to think about warp and weft. They will rarely affect your work. But it's good to know how your fabric might move, and that's a direct result of warp and weft.
Some types of stitching, such as drawn thread or pulled thread embroidery, involve working with warp or weft threads specifically.
You will also find that some embroidery fabrics have very different patterns of warp and weft, as they are woven specifically for a type of embroidery, such as huck toweling for Swedish huck embroidery.
It's also important to pay attention to this when working on counted-thread embroidery or any type of stitching that requires evenweave fabric. Some linen fabrics look similar to evenweave, and they may even work for your project, but not all linen is created equal. For example, sometimes the weft threads are finer than the warp threads or they may all be different thicknesses. This results in stitching that is not squared or even (Kasuti embroidery evenweave).
Stretch is important to understand since you'll want to avoid distortion when you transfer patterns and place your fabric in a hoop.
When pulled warp-wise, most fabric types will have very little stretch but pulled weft-wise, there will be a small amount of stretch. The warp threads are typically stronger, as they have to run the entire length of a bolt of fabric.
Fabrics with warp and weft threads have the most stretch when pulled diagonally, or on the bias. In some sewing situations, this is helpful, but in embroidery, it can cause distortion. To avoid this, keep an even tension on the warp and weft threads as you are transferring your pattern or putting it on a hoop.
To get the straightest cut of fabric, follow the warp and weft as you cut. With looser weave fabrics you can pull a thread out from the entire width of fabric or length you are cutting, then cut the gap left by the pulled thread. You'll also use this technique for making a self-fringing edge.
Other fabrics will naturally tear along the grain if you make a small snip in the edge and then swiftly rip the fabric.