The unique microquasar SS433 is a massive X-ray binary system at an advanced stage of evolution. The optical star is overflowing its Roche lobe and transfering mass at a very high rate onto a black hole, around which a supercritical accretion disc inclined to the orbital plane has formed with relativistic collimated outflows (jets). Both the disc and the jets precess with a period of 162.3 days. In the outer parts of the precessing jets, emission lines of hydrogen and neutral helium are formed, which move periodically across the spectrum of SS433 with an enormous amplitude of ∼ 1000 Å or on the ∼ 50,000 km s⁻¹ velocity scale. This unique feature of SS433 attracted much attention of scientists in 1979. Over many years of research in the optical, infrared, radio, X-ray, and gamma-ray ranges, many important results have been obtained about the physical processes occurring in this microquasar, but a number of fundamental questions about the nature of SS433 remained unresolved. A 30-year spectral and photometric monitoring of SS433 has been carried out at the Sternberg Astronomical Institute of Moscow University. Using all published data for 45 years of observations, we have obtained a number of important results concerning the nature of this unique microquasar. We discovered a secular evolutionary increase in the orbital period of SS433 at a rate of (1.14 ± 0.25) × 10⁻⁷ seconds per second. On this basis, it is shown that the relativistic object in SS433 is a black hole with a mass exceeding 8 M_☉. It is shown that the distance between the components of SS433 increases with time, which prevents the formation of a common envelope in the system. The size of the Roche lobe of the optical donor star is, on average, constant in time, which ensures stable secondary mass exchange in the system. The ellipticity of SS433's orbit was discovered, strongly supporting the model of a slaved accretion disc tracking the precession of the rotation axis of the optical star, which is inclined to the orbital plane due to an asymmetric supernova explosion accompanying the formation of a relativistic object. The parameters of the kinematic model of the system, except for the precession period, keep constant on average for 45 years. Phase shifts of the precession period were detected, but, on average, the precession period remains constant for 45 years. Microquasar SS433 is physically similar to many ultra-luminous X-ray (ULX) sources discovered in recent years in other galaxies. The registration of hard gamma-ray emission up to 200 TeV from the W50 nebula indicates a possible acceleration to ∼PeV hadrons in the region of the interaction between the powerful wind from SS433 and the matter of the nebula. In SS433, the peculiarities of supercritical accretion onto black holes are most pronounced. Therefore, further multi-wavelength studies of this unique microquasar are very promising.