A lifeline is a length of thread or yarn threaded through a row of stitches. It is typically used by knitters doing relatively complex stitch patterns such as knitted lace. The lifeline is generally inserted through the live row of stitches and knitting continues as if it were not there. If it becomes necessary to rip back, the lifeline stops the unraveling process, helping to ensure no stitches are lost.
The simplest way to insert a lifeline is to use a knitting needle with a small hole or "eye." (This is true of several of the interchangeable circular needle sets.) By inserting the lifeline (dental floss is often used) through the hole in the right needle and then knitting as usual, the lifeline is carried along through each stitch. At the end of the row, pull the needle through until you can pull the tail from the hole in the needle to free it and voila - insurance against catastrophe. Knit on with confidence.
Be sure, when inserting a lifeline, that you do not run it through your stitch markers; if you do, they will be caught on the line, and you'll have to either pull the line out, or abandon the markers and place new ones on the subsequent row. This caution is especially important when automatically inserting a line with interchangeable needles; the easiest path is to drop all the markers on the row where you place the line, and place them again on the subsequent row.
Knitting the row after the lifeline is somewhat fiddly because it requires taking care not to get the lifeline caught up in the stitches.