String Theory basics: Instead of 0-dimensional point-particles, all particles are treated as disturbances on strings, with a mass and spin (like the spin of quarks or electrons that are in the same orbital). This way, since gravitons are treated as having 0 mass and 2 spin, quantum mechanics can be combined with gravity. Why couldn't we do it before? Because 0-dimensional particles would by necessity interact at 0 distance, where Einstein's laws break down, whereas the strings are 1-dimensional, have short lengths, and therefore allow Einsteinian physics to make sense.