Antibodies are part of our immune system, but are in short supply when called upon to help kill cancer cells. So science has tried to "mass produce" key antibodies that come from mice and then are "humanized"; these cloned antibodies then can be used in treating cancer.
mono... refers to a single antibody with very specific cancer-killing properties that is to be mass produced. clonal... refers to a process of duplicating a cell (in this case one with special cancer-killing properties) through a process of special cell engineering. antibody... they are proteins in the immune system that help destroy foreign bodies (like a virus or cancer cells).
mabs are made... from thousands of atoms linked together in a very complex structure, and sometimes called "Big Molecule Drugs". There also are "Small Molecule Drugs" and some of them are used to treat cancer; but they are derived from synthesizing chemicals and typically are made up of less than 100 atoms.