Commercial patterns have marks to help guide with layout, cutting and construction of a garment. One of the most important markings is the grain line. The grain line indicator on a pattern is a long line with arrows at either end. This line is one of the longer markings on a pattern and is used to measure distance from either the fold or the selvage. The lengthwise fibers make up the grain of the fabric and are parallel with the selvage edges.
To make sure your pattern piece has been properly aligned with the grain, measure from the grain line indicator on the pattern to the selvage. Measure near the beginning of the mark to the selvage then again near the end of the mark to the selvage. If your pattern is properly aligned on the fabric, the measurement to the selvage will be the same at both of the measuring points. This simple measurement can make all the difference in how a garment is constructed and how well it will wear on the body.
Sometimes pattern pieces are placed on the fold. Place on fold lines are usually on the center front or center back pattern pieces. This is usually a pattern piece that you only need to cut in one piece instead of two. It is easy to make a symmetrical cut when placed on the fold. Pieces that are placed on the fold do not get cut along the fold line. The fold will also be straight with the grain and parallel to the selvage.
*From Online Etymology Dictionary