what does these lines mean:

"Gainst the hot season;the mid forest brake,rich with a sprinkling of fair musk-rose blooms;"

Keats had tried to write an epic poem on the lines similar to Milton. The line mentioned in the question has been taken from his poem "Endymion". Although, Keats was not able to complete the epic, it has many unforgettable lines. The particular line describes the beauty of nature. The "Endymion" begins with the poet hailing Nature and her bounty. He says that each an every object in Nature, be it the daffodils blooming in the grassy meadow or the quiet flowing stream, the sun, the moon or other celestial bodies all make Nature beautiful. In spite of the heat of summer when the ground is covered with thick moss, the lovely, fragrant musk rose blooms. According to Keats, the diversity of Nature lies in the fact that two contradictory things can co-exist together. The particular lines provide us with a picturesque perception of Nature with all her beauty.