A life line is a piece of yarn or floss or I've even heard of people using fishing line that you thread through a row of your work after you have worked it and it makes it so that if you mess up and have to frog you have a row that ready. Just mark where you put the life line so you can start from there again.
If you are using interchangables you can also run it across your cable part and in the holes (the ones that you use the tool to tighten them) and just knit and it will just knit onto your lifeline.
Amy has a video on it here
it's down under fixing mistakes.