For Better and For Worse

Most people, upon marrying, dwell heavily on the portion of the ceremony which goes a little something like this: For better or for worse, in sickness and in health, blah blah blah, until death us do part. The specific area of focus is the death part, usually.

My current obsession (much thanks to my best friend who got me thinking about it) is "for better or for worse."

What does it mean? I mean, what does it REALLY mean? To me? What IS the "for worse" part, and how worse does it have to get before it gets better? Does this mean if I am unhappy in my marriage for some fundamental reason, I am to stick with it and grit my teeth and be happy with my lot in life? How much of the "for worse" do I endure before I am okay to call it quits and move on with my life? What is worth surviving, and what is the payout?