Decreasing is a general term for a technique that reduces the number of columns of stitches a section of knitting has. The decrease can either be right-leaning or left-leaning (referring to how the final appearance of the right side of the knitted section looks, after the decrease). A pattern will likely either tell you the decreasing stitch to use at a particular place in the pattern, or tell you to "decrease evenly" across a row, and how many stitches to decrease. Since decreasing or increasing all at the beginning or end of a row would lead to puckering, you would want to spread the decreases out over the length of the row. Decrease once every N stitches, where N is the number of stitches in the row divided by the number of increases. This will likely not give you an even number; simply round down and knit one extra stitch before decreasing a few times throughout to get the appropriate number.