A real lifeline is a length of yarn or thread that's inserted, left to right, into the stitches on a "particular row" that you've designated as a "base".
For example, I'm working on the Japanese Feather shawl at this time. It's a 28 row repeat. The first 14 rows swerve to the right, the last 14 rows meander to the left. Rows 11-14 and Rows 25-28 are plain st st. So Row 14 and Row 28 are good rows in which to install a lifeline. If I screw up in the lacy rows, and it's almost impossible to fix a mistake after the fact...then I can take my work off the needles entirely, frog back to my last known (base) row...and the lifeline that's threaded into that row will prevent me from losing any of those stitches. And since there are a lot of yarnovers and ssk's and k2t's...it really hard to frog back and expect that all the stitches on the needle will cooperate.
Here is a photo of my Japanese Feather shawl with the Row 14 and the Row 28 lifelines installed. (Under normal circumstances, a repeat is not 28 rows, much less really, however this pattern has 28 that are divided into two, like I said, meandering to the right on Rows 1-14, and meandering to the left on Rows 15-28. If I screwed up on Row 18, I'd just frog back to base row 14 instead of waaaay back to Row 1 of the entire thing)
Icy Blue marks Row 28. Yellow marks Row 14. (different colors are a "must" because I can't tell which Repeat I'm in by eyeballing it)
This is my thread. It can be any type of thread, or any yarn skinnier than your project yarn. My thread is easily installed, easily pulled out. After I've finished Row 28...and then Row 14 again...I'll pull out the yellow from a previous Row 14 and reuse it in the current Row 14. Kind of like "leap frog". I don't remove any lifeline until I feel safe that I've worked the current repeat correctly.
Most people install their lifelines in the last row of the repeat, the WS row which doesn't usually have any fancy work like yarnovers, etc.
Pretend that a Row 28 is just finished knitting, and is still on the source needle. This is how you install the lifeline, left to right, pull through, leaving long tails sticking out at each end so that if the work stretches or flexes on the needle, the lifeline won't come undone at the end, shrinking and leaving 6-10 stitches naked and unprotected.