CQI stands for Channel Quality Indicator. UE send CQI to eNodeB over PUCCH or PUSCH channel, the value range between 0 -15, where 0 means invalid or out of range. In CQI UE tells to eNodeB about downlink channel condition, also request if eNodeB used this coding scheme UE would be able to demodulate/decode data better. Higher CQI value means eNodeB can be use modulation scheme which would enable higher data rate.