Solar neutrinos led to actual breakthroughs on the neutrino, Sun and star physics.

The study of very low-energy neutrinos requires a suppression of the natural radioactivity up to ultra-traces level. This is what the BOREXINO experiment succeeded to achieve with the development of $ad hoc$ technologies, allowing a threshold down to $100 {keV}$. So, the entire solar neutrino spectrum has been detected, together with a small, but clean and robust geoneutrino signal, providing new achievements in the solar, stars and neutrino physics, as well as opening new horizons in the study of the Earth's interior. Among the results, actual breakthroughs can be counted: the identification of the nuclear reactions which produce $99%$ of solar energy and then the solar luminosity, with also the demonstration of the solar stability on a $10^{5}$ years scale; the independent demonstration of the validity of the MSW neutrino oscillation solution, through the determination in a single experiment of the transition of the electron neutrino survival probability from the vacuum to the matter regime; the first experimental detection of the CNO cycle, which dominates in the massive stars; new developments in the solar physics. Borexino is still now a unique experiment with its unprecedented radiopurity level, and is a reference for the very low-energy neutrinos study.