Preloading data into a reserved area of memory (the buffer). In streaming audio or video from the Internet, buffering refers to downloading a certain amount of data before starting to play the music or movie. Having an advance supply of audio samples or video frames in RAM at all times prevents disruption if there are momentary delays in transmission while the material is being played. Even a live broadcast would have a few seconds of delay built in. See streaming video
and adaptive streaming
Buffering Over the Internet
If a network is fast enough to keep up with playback, buffering is not necessary. However, this is not the case over the Internet where packets can traverse numerous routers from source to destination, and delays can be introduced at any juncture. See double buffering
and buffer flush
Filling the Buffer
The message means 70% of a reserved area in memory is filled at that moment. When it reaches 100%, the software (Windows Media Player in this example) will start "playing" the video.