Have you seen the [size=6][color=blue]video[/color][/size] on this site?
It’s on the Basic Techniques page. Mouse down to “More” & it’s about halfway down the page.
Basically, it’s a way to rip back to a specific point in your work if you realize you’ve made a mistake, rather than back to the beginning. (Maybe the first row of a repeat would be a good place…that way you’d know which row to begin at again.) It’s done because it’s hard to remove stitches from the needles to rip back say 8 or 10 rows & then be able to gather them all back up again…because of the amount of yarn overs & stitches knitted together and such in a lace pattern.
When you finish a pattern repeat & have checked to make sure it looks like it ought to, you can either put in a second lifeline at the start of the next pattern repeat and then alternate moving them up, one at a time, or remove the one that’s there & use that.
The lifeline itself can be anything…a contrasting color piece of yarn, a piece of fishing line, one of the extensions from an interchangeable needle set, anything that’ll hold the stitches without stretching them.