ABR (adaptive bitrate streaming) is the technique by which the player switches between renditions in real time to match the viewer's changing network speed. Instead of committing to one quality, the player continuously measures throughput and buffer level and, at each segment boundary, chooses the rendition that keeps playback smooth — climbing to higher quality when the network is good and dropping down before the buffer empties.
ABR is what makes internet video watchable on unpredictable connections; it is the reason the encoding ladder and the manifest exist. The decision lives in the player's ABR algorithm, which balances three goals: avoid rebuffering (the worst outcome for QoE), maximize sustained quality, and minimize disruptive quality switches. Algorithms range from throughput-based to buffer-based to hybrid (e.g. BOLA, MPC).
Good ABR is central to QoE: it directly governs startup time, rebuffering, and average bitrate. Tuning it — and the ladder it chooses from — is one of the highest-leverage things a streaming team can do for perceived quality.
