Neutrinos are produced and detected in flavor states—electron, muon and tau—but these are not states of definite mass. A flavor state is a quantum superposition of mass eigenstates: $$|\nu_\alpha\rangle=\sum_i U_{\alpha i}^*|\nu_i\rangle,$$ where $U$ is the neutrino mixing matrix.

Different mass eigenstates accumulate different phases while propagating. When they are recombined in a flavor measurement, interference changes the probability of detecting each flavor. In a two-flavor approximation, $$P(\nu_\alpha\to\nu_\beta)= \sin^2(2\theta)\sin^2\left(\frac{\Delta m^2c^3L}{4\hbar E}\right),$$ where $\theta$ is a mixing angle, $\Delta m^2$ the difference of squared masses, $L$ the propagation distance and $E$ the neutrino energy.

Oscillation therefore depends on the ratio $L/E$. A beam initially produced almost entirely as one flavor can arrive with a different mixture after traveling far enough.

Observation of neutrino oscillations proves that at least two neutrino mass eigenstates have different nonzero masses. This goes beyond the minimal original Standard Model in which neutrinos were massless.

Oscillation is a coherent quantum interference effect, not a neutrino physically changing identity at a particular point. The propagating state is a superposition; flavor is the basis selected by weak-interaction production and detection.