Increasing is a general term for a technique that adds to the number of columns of stitches a section of knitting has. A pattern will likely either tell you the increasing stitch to use at a particular place in the pattern, or tell you to "increase evenly" across a row, and how many stitches to increase. Since increasing or decreasing all at the beginning or end of a row would lead to puckering, you would want to spread the increases out over the length of the row. Increase 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 increasing a few times throughout to get the appropriate number.