• Neutrino oscillations are a consequence of quantum nature of neutrinos along with all other elementary particles.
• Quantum mechanics suggests that all elementary particles  (therefore all matter) have dual nature and dual properties:
• wave-like properties and
• particle-like properties.
• One property of the wave state is the principle of quantum superposition. The principle of quantum superposition states that even though we can measure a system as having a defined set of characteristics, multiple states can coexist in the same wave.
• These quantum mechanical concepts were applied to the solar neutrino puzzle. The electron neutrinos born in the sun are a superposition of three neutrino wave states with definite masses.
• Different mass wave states travel at slightly different speeds and the combination (superposition) content  slowly changes along travel in space. As a result, at different points along travel the three mass wave states combination (superposition) is slightly different, looking  like muon neutrino or tau neutrino (mass wave states superposition). 
• *We refer to different neutrino types as neutrino flavors.

The states oscillate back and forth, so this phenomenon is called neutrino oscillation.