Mass number of isotopes of copernicium denoted with 'm'
The 'm' does indeed stand for metastable. The word 'isomer' in this sense refers to nuclear isomers, in which at least one of the nucleons is in an excited state. Think of it as somewhat analogous to an electron in an excited state, but occurring within the nucleus. Just like electrons release photons when an excited state decays to the ground state, when nuclear isomers decay to the ground state (which is different than 'normal' nuclear decay like $\alpha$ or $\beta$ decay), a gamma ray is release.