A life line is a piece of scrap yarn that you run through your last set of stitches when you know the knitting before that row is correct.
What it does is give you a safe space to rip back to if you make a mistake. It prevents you from having to rip all the way back to the beginning.
You just take a piece of scrap yarn and a tapestry needle and just thread it throught all your stitches on the needle then you can proceed on to knitting the next row. You can insert life lines as often as you'd like so you don't have to rip back as far. Only leave one in at a time though or it may distort the stitches.
The white pieces of yarn in the picture below are life lines.