Shakespeare's Sonnet 18 is ultimately about the poet's belief that his "eternal lines to time" (this poem) will allow the person he is speaking to to live forever. He claims in the final couplet that as long as people are reading this poem "so long lives this, and this gives life to thee." The "this" that he is referring to is the actual words/language of the poem.
Now that you know what the poem is actually about, the third quatrain actually makes a bit more sense in the context of the conclusion. The poem opens with two quatrains of comparisons explaining how his love is better than a summer's day. The third quatrain claims that the beauty of the beloved will never fade (like real summer does), and Death will never have a chance to brag that the beloved is with him--not because the beloved will never die, but because even though the person will physically die, they will never be dead and gone because they will live on through this poem. As it turns out--Shakespeare was right! Here we are nearly 400 years later, taking about the beauty and life of his beloved.