The Rabi frequency is a measure of the oscillation rate of a quantum system when it interacts with an external electromagnetic field. It describes how quickly a two-level quantum system, such as an atom or a molecule, can transition between its energy states when exposed to a resonant frequency of light or radio waves. In practical terms, the Rabi frequency depends on the strength of the external field and the properties of the quantum system. A higher Rabi frequency indicates a stronger interaction, leading to faster transitions between states, which is essential in applications like quantum computing and nuclear magnetic resonance (NMR).