A lifeline in knitting is a loop of some yarn or thread that goes through a row of stitches so that if a stitch is dropped it can not unravel past the row with the lifeline.
What you need:
your knitting that you want to protect and appropriately sized needle and yarn or thread. I used this pink thread because it was sitting around. Another lace weight yarn would have worked very well also.
Lay your knitting flat, and thread your sewing needle through all the live stitches. Just follow where your knitting needle is. (Be careful not to go through any stitch markers.)
When you have your yarn or thread through all the live stitches tie the ends together so it won't fall out. Now if you drop a stitch, it won't fall past the lifeline. If your knitting comes of the needle everything below the lifeline is protected.
How often should you use a lifeline? Think about how upset you would be if you lost that portion of your knitting. The more upset you would be the more often you should use the lifeline, saving you the heartache of having to reknit that section should tragedy strike.
Here I've added another lifeline with a yarn that stands out a bit better.