Quality of service refers to mechanisms used to manage how different traffic is treated under network constraints. Supported equipment can classify, prioritise or otherwise schedule traffic according to its configuration. QoS does not create extra internet capacity, and settings on one home device cannot guarantee treatment by every other network along a media delivery path.
A practical example
A router may let a household give a viewing device priority while a local connection is busy. Whether this helps depends on the actual bottleneck, correct configuration and the router's supported scheduling features under those conditions.
A common misunderstanding
A priority label is not a promise of uninterrupted viewing or guaranteed treatment by an internet provider. Measure the effect and avoid applying unexplained rules to every device.
Related terms
Further reading
Stable technical definition. Editorial approach