Under-served describes a group of people, or the place they live, that does not get enough of some service — health care, schools, banks, buses, shops, internet. The word does not say anyone did anything wrong on purpose; it just says the supply is too thin for the need. You hear it most from governments, charities, hospitals, and companies, and it is a fairly formal, report-and-policy sort of word rather than something you say at dinner. It usually points at whole groups or areas, not one person: an under-served community, an under-served rural district, under-served students. Businesses have borrowed it too, in a hopeful way — 'an under-served market' means a lot of customers nobody is selling to yet, so it is an opportunity rather than a sadness. Written both as under-served and underserved; both are fine, and the one-word spelling is more common in American writing. There is a matching verb, to underserve, but it is much rarer than the adjective.