Neutrinos are extremely light neutral particles; for a long time, it was believed that they were massless. Now we know that they have a very small mass and that they can be of three types or flavours.
The peculiarity of these particles is that they very rarely interact with matter. In fact, neutrinos pass undisturbed through any object they meet (almost) without leaving a trace: even now, while you are reading, 100 billion neutrinos per second are passing through your thumb, but on average, only one neutrino a year might interact with it! Capturing neutrinos is a very arduous task and can only be done in particular circumstances, such as in places that can be reached only by neutrinos; it is for this reason that laboratories are built at a great depth underground, under the Gran Sasso Massif, for example.
The curiosity around neutrinos increased when the phenomenon, known as neutrino flavour oscillation was discovered: in fact, in certain conditions, neutrinos are able to change flavour as they travel. In short, neutrinos seem to be phantom particles with an inclination for transformism.
Neutrinos can be of both natural and artificial origin. The Sun is a powerful source of neutrinos that are emitted by the nucleus during the nuclear fusion reactions. Since they rarely interact with matter, these particles are able to escape the Sun immediately, in contrast with light that takes 10 million years to reach the surface. Neutrinos are a very important subject of study, being the only carriers of direct information regarding what occurs in the nucleus of our star.