LOADING:
Consider a simple case in which you want to supply the whole V(O) to the load resistance R(L). Now this can only take place when either R(O)=0 or when R(L)=∞. Since both of the cases are impossible to achieve practically so whole V(O) can never be supplied to load resistor. This phenomenon is called loading. Loading is inherent in any practical circuit available. Only way to decrease it to maximise the load and minimise the internal resistance R(O).
Loading can occur in many ways but in the core of all types of loading is a other resistance coming in parallel or series with the load.