Complex sentence: The song is about a monk, brother Jacob, who has overslept and is urged to wake up and sound the bells for the matins, the midnight or very early morning prayers for which a monk would be expected to wake.
Split into simpler sentences: The song is about a monk, brother Jacob, who has overslept and is urged to wake up and sound the bells for the "Matin", the nighttime liturgy. The Matins mentioned in the literal translation refers to the midnight or very early morning prayers for which a monk would be expected to wake.