Carrying is one way of doing colorwork in crochet; it is also called tapestry crochet.
In this method, another color is “carried” along the top of the previous row, with new stitches in the current color being worked over it. The off color becomes locked inside of the current color stitch, hiding it from view. Using this method, it is easy to switch back and forth between colors to form designs, only showing the color you want.
Follow along with the video for a quick, concise illustration of this useful technique.