A lifeline is a piece of contrasting yarn that's treaded through a row you know is right, so if you have to rip back you only have to go that far.
There's a demo under basic techniques - more - fixing mistakes in the video section of KH. It's a great idea, especially on complicated patterns or lace. I just use a tapestry needle and lightweight yarn like fingering weight, or sometimes even heavy button thread, and slide it along the bottom of my needle on the finished row.
When you get farther into the pattern you just put in another lifeline closer up to the top of the work and pull the old one out.
It's also great if you have to take your knitting off the needles for some reason!
Happy Knitting :XX: :XX: