To have clear understanding of RI, you have to understand the detailed concept of MIMO, Channel Status Information Matrix and mathematical definition of Rank of a matrix. Depending on instruction from the network, UE may periodically or aperiodically measure RI and report it to Network. Refer to CQI,PMI,RI Report section for this.
But I will explain the practical/intuitive meaning of RI here.
In simple words, RI is an indicator showing how well multiple Antenna work. What do you mean by "how well the multiple Antenna work ?". We usually say "Each of the multiple antenna (e.g, each antenna in MIMO configuration) works well if the signal from each antenna has NO correlation to each other". "No correlation" implies "no interference to each other".
Maxmum RI value is very closely related to the number of Antenna. Maximum RI is same as number of antenna on each side if the number of Tx antenna and Rx antenna is same. If the number of Tx and Rx are different, the one with less antenna is the same as Max achievable RI.
Max RI means "No Correlation between the antenna", "No interference to each other", "Best Performance".
For example, in case of 2x2 MIMO, the RI value can be 1 or 2. When the value 2 in this case means "No Correlation between the antenna", "No interference to each other", "Best Performance". If the value is 1, it implies that the signal from the two Tx antenna is percieved by UE to be like single signal from single Antenna, which means the worst performance.