The buffer is the cushion of media the player has downloaded ahead of the current playback position. When the network briefly slows or drops, the player keeps drawing from the buffer so the viewer sees no interruption; rebuffering happens only when the buffer runs dry.
Buffer size is a central design trade-off. A larger buffer is more resilient to network variability but increases latency (especially bad for live) and memory use, and wastes bandwidth if the viewer abandons. ABR algorithms watch buffer level as a key signal - filling it when conditions are good and easing quality before it empties. Tuning buffer targets is one of the most direct levers on the rebuffering that drives QoE.
