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.
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.