'Mono' means 'one' and 'clone' means 'identical copy'. Monoclonal antibodies are identical copies of one type of antibody.
Antibodies are proteins produced by a type of white blood called lymphocytes. Pathogens have proteins on their surface called antigens. When a pathogen infects the body, the lymphocytes recognise these antigens as foreign and attack them by producing antibodies.
Antibodies bind to specific antigens on pathogens. This means that only one type of antibody will bind to a matching antigen. Scientists discovered that we could make antibodies to bind to antigens on other substances, and not just those on pathogens. Once bound, the antigens - and the substances they are found on - are merged tightly together. This makes them easier to identify and deal with.