The term substrate level phosphorylation refers to the synthesis of ATP by reactions in which ADP is one of several substrates and ATP is one of several products of an enzyme catalyzed reaction.
In the example below the substrates are phosphoenolpyruvate and ADP. These two molecules bind to the active site of the enzyme pyruvate kinase. During catalysis, the phosphate group of the phosphoenolpyruvate is transfered to the ADP molecule producing ATP and pyruvate. Thus the products of the reaction are pyruvate and ATP. This enzyme driven reaction is, of course, reversable under certain conditions of substrate and product concentrations.