Protein phosphorylation is a reversible post-translational modification of proteins in which an amino acid residue is phosphorylated by a protein kinase by the addition of a covalently bound phosphate group. Phosphorylation alters the structural conformation of a protein, causing it to become activated, deactivated, or otherwise modifying its function.[1] Approximately 13,000 human proteins have sites that are phosphorylated.

Reference
Wiki: Serine phosphorylation