The selvage is a finish on the edge of the fabric that binds the edge of the fabric and is often a different color than the textile. The selvage runs parallel with the lengthwise grain (warp) of the textile and is very important to become accustomed to when starting your endeavors in clothing construction. The threads that run parallel with the selvage are the strongest in terms of resisting stretch. The lengthwise grain is most often placed vertically in clothing to prevent any or sagging in your garments.
On the contrary the crosswise grain (weft) are the fibers that run perpendicular to the selvage. These are the fibers that weave between the lengthwise grain. Think of fabric on a loom. First, fibers are arranged taught vertically up and down on a loom (lengthwise grain), and them horizontal fibers weave in and out, in and out, of the vertical fibers. Since the crosswise grain is taking a less direct path between the lengthwise grain bending and curving, they will have some give to them. This makes the crosswise grain a little more forgiving will have a little bit of stretch. We must be careful not to orient our patterns on the wrong direction or else you will have a bit of stretch and sagging where it is not ideal. Luckily, having a bit of stretch in one direction (around our circumference) is pretty ideal when it comes to clothes, how cool is that?!
Another thing to note when it comes to fabric is the true bias. This is not technically a grain but a characteristic of fabric when cut on the diagonal. Fabric cut on a diagonal will have more stretch along the cut edge compared to the crosswise/lengthwise grain. True bias is when it is cut exactly at a 45 degree angle to the selvage (or intersects the warp/weft at a 45 degree angle). When you cut out a pattern, note that any cut line cut at an angle will be inclined to stretch a little so be careful with your cut pieces.