Killing time is the pursuit of activity that is designed primarily to occupy your mind while waiting for time to pass.
Ideally, this is because something important is going to happen soon and you don't have the organizational skills to get something useful done in the interim. More likely however, if you're killing time, you've fallen into some habitual time-sink like watching Friends. You may have even deluded yourself into believing that you're doing it because it is really quite enjoyable but the truth is that you just don't have enough drive to do something genuinely interesting.
It seems that most people I encounter live a life of three modes: work, sleep, and kill time. While I find this very sad (like if I think about it enough, I could start crying at the waste of potential), it's less stressful to be snide and holier than thou about it. Life isn't a spectator sport.
Imagine what we could do if we took all the time wasted by people waiting for something to happen and directed it toward feeding and educating the impoverished people of the world, or reaching space in a serious way, or even just noding cool stuff. The inhabitants of the great North American Game Preserve have lost their way.