what does the 2nd line in sonnet 55 means

This is another sonnet that addresses his beloved. Exactly who that beloved was is open to speculation. The line you are referring to has to do with the poet's confidence in his own talents as a poet. He says that the power of his prose will keep his beloved's memory alive forever, or at least longer than gold plated shrines, "Not marble, nor the gold-plated shrines of princes, shall outlive this powerful rhyme."