A lifeline is a piece of waste yarn that allows you to rip back your stitches to the start at the beginning of a repeat when knitting lace.
tutorial - inserting an afterthought life line - La Visch Designs
Tutorial: beginner's lace, Blog
Adding a life line to your knitting
Knitting lifelines: what they are and why you need them! - Gathered
Lifeline - What? Why? How? - 10 rows a day
How to Use A Lifeline
How to Use a Lifeline - Knitting Stitch Dictionary
Knitting Brioche Stitch - Yarnalia
How to Knit With a Lifeline and Prevent Disasters - A Bee In The Bonnet
The difference between tinking and frogging