A lifeline is a length of constrasting thread or yarn threaded through a row of stitches in a knitted piece in progress. Any dropped stitches can only unravel down as far as the lifeline, and if you need to rip back because you've made a mistake, it's easy to rip back just to the lifeline and get your stitches back on the needles..
Dental floss and viscose crochet thread make good lifelines because they're thin and slippery and don't give off any lint.
You can add a lifeline on the current row by threading a sewing needle with your lifeline thread and then working it through each stitch.
If you need to rip back to a given row but haven't got any lifelines, you can often carefully thread a lifeline through that row, again using a sewing needle.
Lifelines are perhaps most useful in lace knitting or other complicated patterns.