Final answer:
Shakespeare is using a metaphor to describe love as a bright, guiding light in lines 84 and 85 of Sonnet 18.
Step-by-step explanation:
Shakespeare is using a metaphor in lines 84 and 85 of Sonnet 18. He compares love to a bright, guiding light. This metaphor suggests that love is constant and illuminating, just like a guiding light. It also implies that love provides warmth and guidance, similar to a guiding light in the darkness.