"Buffering" is the act of downloading enough of the media so that you can start watching asap. It's the alternative to downloading the entire clip and starting when that's finished.
When the buffering starts, the playback plugin attempts to calculate the minimum initial download needed so that you can watch to the end without interruption (it downloads the rest during your viewing).
There are several factors that can affect this, unfortunately not all of them are "in your house" and therefore in your control. The most common cause is another download starting (or speeding up) somewhere on the network that temporarily affects your connection speed. This might be on your computer, on a computer in your house, or on a neighbour's computer. Not a lot you can do about it unless you go "hands off" on your own PC, tell the kids to stop downloading their iTunes, and go out and shoot all your neighbours.
