This type of knitting yarn changes colors or patterns as you knit. The yarn industry has really evolved. The machinery today allows yarns to be dyed in patterns.
When knitted these yarns can provide a variety of interesting patterns. Self striping sock yarns, for example, may have stripes or a Fair Isle pattern when knitted. What's unique is that the yarn looks like you've changed colors, but it all comes from one continuous strand of yarn.
This type of yarn is very popular for knitting socks, baby sweaters, hats, shawls, mittens, gloves, and leg warmers. Used alone, you'll see the colors change throughout the project. It's also very popular to alternate this yarn with stripes of a solid or another striping shade for really unique color-changing results.