A lifeline is a scrap piece of thread or yarn, threaded through your work, as a holding point.  It is used to protect your work from dropped stitches, and if you have to frog your work back if you have an error.

Here is a youtube video by Knitpicks.com that goes through exactly what it is and why a lifeline is important:

I know they say it is for lace knitting, but it is just as important for any large project.  I was bad and forgot to put a lifeline in a project I was working on in June, and I had to frog the entire project.  I had an error 15 lines back, and I tried to knit back the 15 lines, but I dropped 3 stitches that ran back another 10 lines each or so. By the time I got them, I looked at the project and discovered that it didn’t look like it was supposed to. I had put more than 30 hours worth of work into that project (NOT including the 2-3 hours it took me to knit back), and it was ruined because I forgot to use lifelines.  I still haven’t recast it on yet.

So, I’m begging you, use lifelines! Don’t make my mistake and lose your work because of it.