Certain Evangelical Protestant sects believe that the "end times" (the Second Coming, etc.) are near at hand. They have certain specific beliefs about how the "end times" will unravel, involving events such as "the Rapture" (the removal of "saved" Christians from the world prior to all kinds of bad shit happening), the coming of the Antichrist, etc.

I actually don't know of anything to indicate that Palin is an end-timer. Not all Evangelicals are equally obsessed with the Apocalypse. But I will say that the reason end-timers in politics are disturbing is that they tend to see events that are potentially devastating in worldly terms as acceptable and even desirable, in that they are signs of the end. It gives people like this a tendency to be more accepting of war and violence than is healthy for the country. Palin in her comments about the troops needing to be on a mission from God is exhibiting a similarly disturbing attitude for a similar reason, whether or not she should be called an end-timer.