iTunes buffers content when you previewing songs or videos. Until the buffer fills, the content doesn't start playing. And if the buffer goes dry while it's playing, the preview stops.

So if you have a great internet connection, you can get stuff to play with less delay by setting the buffer to be smaller. If you have a bad internet connection and you want to make sure the buffer does not run dry, you can ensure this by setting the buffer to be larger.

You can set this by getting to the Preferences in iTunes and then going to the Advanced tab. There is an entry called "Streaming Buffer Size" and this is the one you'll want to adjust.