In video streaming or audio streaming from the Internet, buffering refers to downloading a portion of the data before starting to play the movie or music. Having an advance stream of video frames or audio samples in RAM always avoids interruption if there are temporary delays in transmission while the content is being played. Even there would have a few seconds of delay built in for a live broadcast.
For instance, when you try to play a video or audio file from the Internet, it may load the initial 10% of it into a buffer and then start to play. The device constantly downloads the rest of the content and stores it in the buffer while the content plays back. When there is network congestion, there is fewer of a chance that the video or audio will skip or stall as the content is being played from the buffer, not directly from the Internet.