Some elements have "m" isotopes with the same mass number. What does the "m" mean?

"m" stands for metastable.

Metastable refers to the state of a nucleus , where nucleons (subatomic particles which make up the nucleus, i.e. protons and neutrons) are excited and thus exist at different energy states. with the same nucleons (and thus same mass number) but differ in energy (and thus differ in the manner of radioactive decay) are known as nuclear isomers.