Fair Isle is a specific form of colorwork knitting named for Fair Isle, a small island off the coast of Scotland where knitters were known for their skill with the technique.
Fair Isle traditionally uses two colors of yarn in a row, though more than two colors can be used in a project. The yarn that is not being worked is carried along the back of the work, creating strands or floats that make the knitting twice as thick as it would otherwise be.
The knitting of Fair Isle was popularized when the future king Edward VIII was seen sporting the designs while golfing in the 1920s.
These days, Fair Isle is sometimes used to refer to any stranded knitting technique in which two colors are worked per row. Knitting history purists say that Fair Isle should only refer to those patterns that are traditional to the Shetland Islands where the technique originated, calling other motifs using the same stranding technique stranded colorwork or stranded knitting.
Whichever term you use, Fair Isle or stranded knitting is a quick and easy way to add color to otherwise basic Stockinette Stitch projects.