Petrarch's Sonnet 18 is about Laura, her beauty and Petrarch's incapability to describe her beauty in a proper way.
His love for her is related in this sonnet. He is continually praising her beauty
"When first I saw thee I recall the time,
Pleasing as none shall ever please again."
"...Full oft I oped my lips to chant thy name..."
It is also a poem about defeat. He uses repeatedly negative words and phrases to state clearly that her beauty is impossible for him to describe, "unsung...in my rhyme". He hasn't got any possibilities to make a proper description of her beauty, nor to describe her brightness.
"But ah, the pen, the hand, the vein I boast,
At once were vanquish'd by the mighty theme!
He uses negative words and phrases to strengthen the idea of his inability to make a suitable portrayal of her because her beauty is such that it exceeds his chance to describe it. He hasn't got the strength nor the genius.
