This refers to the assumption that data transmission rates, error rates, and other characteristics can be measured, improved, and to some degree, guaranteed in advance. Basically, QoS describes a collective measure of the level of service a provider delivers to its customers or subscribers. It is commonly used to compare against speed.
For example, Wi-Fi has the speed but not the QoS, and HomeRF has the QoS but not the speed. Service providers may guarantee subs a particular level of QoS defined in a service-level agreement.