These are a type of antibody, a protein produced by the immune system to fight off pathogens and other foreign substances or substances the immune system perceives as a threat.
Deeper Dive
Neutralizing antibodies specifically defend the body against pathogens and can be induced by a vaccine or the presence of a pathogen in the body. They are made by B cells in the bone marrow and operate differently depending on the type of pathogen. For viruses, they bind to the virus to prevent its entry into the cell, its ability to change structure or shape, and/or its ability to replicate. For bacteria, neutralizing antibodies block the effects of toxins produced by the bacteria. You can read more about their specific functions here.
For some diseases, neutralizing antibodies can provide lifelong protection against the disease, such as with vaccines against measles and polio. Other times, neutralizing antibodies can wane or become less effective over time, such as with many respiratory viruses like COVID-19 and influenza. (In the case of flu, it usually just mutates into a different strain that the antibodies no longer recognize.) Measurement of neutralizing antibodies is commonly an endpoint in vaccine studies to assess how effective the vaccine is.