The rate determining step is what part of a reaction?
The slowest step in the reaction mechanism.
Many reactions can involve multi-step reaction mechanisms. Often it is the case it is broken down into one fast step and slow step which might first create an intermediate and then produce the final product, let's say.
The slow step is also called "the rate-determining step". However, the rate expression is not always showing the reactants in the slow step. Sometimes the slow step is dependent on intermediates produced in the faster step, and the rate law based on the slow step might need to be rewritten based on only reactants.