PMI stands for Pre-Coding Matrix Indicator and it is signaled by UE to eNodeB to improve the downlink signal to noise ratio.
PMI is sent to eNodeB using PUSCH/PUCCH channel. Based on number of antenna ports and value of rank indicator, range of PMI varies.
PMI is applicable for closed loop transmission modes such as TM-4, TM-5 and TM-6.
Now the signaled value from UE is used by eNodeB while doing pre-coding.