Updated: April 15, 2017
Neutrinos oscillate, and their oscillations are different when propagating in matter than in vaccum. The two oscillation frequencies are called '12' or solar, and '13' or atmospheric.
The length of the blue line is the probability amplitude for each of the two sectors. The amplitude squared gives the probability.
The horizontal and time axis scale with the matter potential (which is proportional to the neutrino energy). As the potential increases, at a certain point, shows the resonance when the probability hits a minimum.
See arXiv:1604.08167 for more information.