It has to do with the wavelike nature of neutrinos. Waves oscillate back and forth. Add two waves together and you get a new composite wave.
For instance, when two very similar musical notes are played together, there's an interference effect that causes the sound to wobble between loud and soft, producing "beats." Similarly, oscillating neutrinos are comprised of three different waves that combine in different ways as they travel through space. The "beats" are caused by small physical differences in mass that lead to those telltale interference effects.
Scientists can also observe neutrino oscillations in particle accelerators. Physicists have been studying this phenomenon for the last 15 years or so, but while several experiments clearly showed evidence for neutrino oscillations as they travel long distances through space, they still wondered: Could, say, an electron neutrino emerge from a beam of muon neutrinos through the oscillation process?
Last year, researchers at the OPERA experiment at Gran Sasso National Laboratory made the first direct observation of a tau neutrino emerging from a muon beam - another very rare event in high-energy physics. But T2K actually started with a controlled beam of muon neutrinos and detected the electron neutrinos produced through oscillation, offering compelling new evidence in support of the phenomenon.