What does Rayyan mean?
From the triliteral root r-w-y (ر-و-ي, 'to be quenched with water, irrigated'), meaning 'luxuriant, well-watered, lush'. Ar-Rayyan is also the name of one of the gates of Paradise through which those who fasted are said to enter, which drives much of the name's popularity.
What does Bilal mean?
From the root b-l-l (ب-ل-ل, 'to moisten, to wet'), understood as 'moisture, wetting, freshness'. Famously borne by Bilal ibn Rabah, the Abyssinian companion of the Prophet and Islam's first muezzin.