A lifeline is a strand of yarn that is inserted into the work so that, if an error is encountered, it is easy to rip back to that point. Lifelines are often used in lace knitting. Leave lifelines in your work until the piece is complete.
To insert a lifeline, thread a tapestry needle with smooth, tightly twisted yarn (such as crochet or perle cotton) and insert the tapestry needle purlwise through each stitch on the needle, catching each stitch but going around any markers. Do not remove the stitches from the needle. When working on a circular needle, it is easier to insert the lifeline if the stitches are pushed onto the cable portion of the needle. If you must rip back, use a smaller needle to pick up the stitches from the lifeline thread, and then resume working with your regular needle.