Arpit Dhuriya Derive the given string using right most derivation. This will give you steps of how to obtain the string from the given grammar. After that follow the derivation in reverse order; you'll see that as you climb up the steps of derivation - some part of the string in step i will reduce to a one of the LHS of the given grammar in the step i-1. The reductions which you are making in each step while going up are called the "right sentential form of reductions".
This is how bottom up parsers work. They start from a given string and reduce it to start symbol. Sentential form is a fancy name for the intermediate results you obtain in the process. Also, this is why bottom-up parsers are known to use "reverse of RMD" and not just RMD.