Depends on what kind of data your plotting. If it is binding, is it one or two site competition? Also, you need to make sure the X-axis drug concentrations are labelled properly. Then you need to analyze the data using some sort of regression analysis (typically one uses non linear for concentration-response studies). Then just flip to the results tab and it should be right there. Also, if you really need help there is a help tab in prism which gives you step by step examples and is very helpful.
The best interpretation for concentration-response analysis on Graphpad prism is to calculate the logEC50 values for your compounds, which is pharmacological more relevant. The software automatically calculates it for you after you plot a 4 parameter logistic equation, for example, of your drug stimulation/ inhibition/ or percentage stimulation/ inhibition responses.