Fluorine isotopes and isomers
Just like electrons nuclei can exist in a number of quantised states. We see evidence of this when, during radioactive decay, a gamma ray is produced. This is a nucleus in an excited state decaying to the ground state, and the excess energy being radiated away as a gamma ray photon. This is precisely analogous to an electron in an atom falling from a high energy state to a lower energy one, and thus producing radiation which we can see in an atomic spectrum. So an isomer of a nucleus is simply that nucleus in a state which is not the ground (lowest energy) state.

Most isomers are very short lived, and thus the gamma radiation appears to be more or less coincident with any other radiation that is produced during a radioactive decay. For instance the $\ce{^{18m}F}$ isomer you mention has a lifetime of 162ns according to https://en.wikipedia.org/wiki/Isotopes_of_fluorine . However the odd one is much longer lived, and for a remarkable case of one that is observationally stable take a look at $\ce{^{180m}Ta}$. 0.012% of natural Tantalum consists of this isomer, and it has never been observed to decay to the ground state - as is usual in such cases symmetry forbids the transition. And what I find even more remarkable is that the ground state itself is unstable with respect to both electron capture and beta emission, decaying to isotopes of Hafnium and Tungsten with a half life of just over 8 hours! See https://en.wikipedia.org/wiki/Isotopes_of_tantalum for details.

Sorry - slightly belated edit, but it occurred to me that isomers do have one very direct use in Chemistry, namely Mossabuer spectroscopy, see https://en.wikipedia.org/wiki/M%C3%B6ssbauer_spectroscopy. In essence very small changes in the energy of the gamma ray that is emitted when the isomer decays to the ground state can be detected by this technique, and can be related to changes in the chemical environment within which the atom finds itself.