Complex sentence: Particles are so rarefied that almost all photons can leave the Sun's surface without interacting with the matter above the photosphere: in other words, the corona is trasparent to the radiation and the emission of the plasma is optically thin.
Split into simpler sentences: Particles are so rare that almost all photons can leave the Sun's surface without interacting with the matter above the photosphere because the corona is trasparent to the radiation. In other words, in the corona, the emission of the plasma is optically thin.