Complex sentence: Each time a port transmit a frame, that port BB Credit is decremented by one, for each R_RDY received, that port BB Credit is increment by one, if BB Credit is zero the corresponding node cannot transmit until a R_RDY is received back.
Split into simpler sentences: Each time a port transmits a frame that port's BB Credit is decremented by one; for each R_RDY received, that port's BB Credit is incremented by one. If the BB Credit is zero the corresponding node cannot transmit until a R_RDY is received back.