The speaker is Lord Montague; the person he is speaking about is his son, Romeo. He doesn't understand why Romeo has been acting as he has over the past few days. Benvolio reports having seen Romeo early that morning, but he was not in a mood to talk and left before Benvolio could reach him.
Montague describes what he has heard about Romeo's early morning activities. In the early morning light, when "the fresh morning's dew" still covers the grass, Romeo has been seen in "the grove of sycamore" on the west side of town. When sighted in the grove, Romeo appears to be crying tears that augment, or add to, the dew, and sighing so deeply in the cool morning air that his breath forms clouds in addition to the clouds already in the air.