Phosphorylation refers to the addition of a phosphate group. In the case of substrate-level phosphorylation, the phosphate group is removed from an intermediate reactant in the metabolic pathway, and the free energy of the reaction is used to add a third phosphate to an available ADP molecule, producing ATP (see image below).

In cellular respiration this happens during glycolysis and the Krebs cycle.

In substrate-level phosphorylation reactions, the phosphate group of an intermediate reactant is transferred to an ADP molecule to synthesize ATP.