Complex sentence: Temperatures inside stars like the Sun are not high enough to allow colliding protons to overcome Coulomb repulsion, but in a certain percentage of collisions the nuclei pass through the barrier anyway by quantum tunnelling.
Split into simpler sentences: Temperatures inside stars like the Sun are not high enough to allow colliding protons to overcome Coulomb repulsion. In a certain percentage of collisions, however, the nuclei pass through the barrier anyway--an example of quantum tunnelling.