For each streaming clip, RealOne Player keeps a "buffer" that acts as a data reservoir. Data enters the buffer as it streams to RealOne Player, leaving the buffer as RealOne Player plays the clip. The buffer helps ensure that lapses of available bandwidth don't stall the presentation. If network congestion halts the flow of data for a few seconds, for example, RealOne Player keeps the clip playing with the buffered data. Your goal is to minimize initial buffering and eliminate rebuffering.
Initial Buffering (Preroll)

RealOne Player buffers a few seconds of data before a clip plays. Also called "preroll," initial buffering is required for every clip. Developing clips that use an appropriate amount of bandwidth keeps preroll to an acceptable level. You want preroll to be low—less than 15 seconds for each clip. RealAudio and RealVideo encoding tools set a low preroll for you. With other clips, though, how you create the clip determines its preroll.
Rebuffering

When clip data has stopped coming in and the clip buffer is empty, RealOne Player has to halt clip playback to store data again, or "rebuffer." Sometimes this is unavoidable because the viewer's available bandwidth drops for too long. When developing a multiclip presentation, though, you need to consider timelines carefully so that you don't inadvertently cause rebuffering, which can happen if too many clips fight for too little bandwidth.