The Block Error Rate (BLER) is a measure of how successful a data transmission is over the air at the Physical/MAC layer level. If a transport block is successful decoded (the CRC calculated by the receiver matches the CRC sent in the block, then the transmission is successful. The target BLER for the radio link is typically set to 10% (meaning that the transmission must be sent in such a way as to achieve a 90% success rate on average).
If the transmission fails, then the receiver indicates this with a HARQ NAK (negative acknowledgement); the sender can then send additional information (e.g. error correction bits) to help the receiver decode the original transmission (a so-called HARQ retransmission).
iBLER is initial BLER where as rBLER is residual BLER after retransmission.