Not yet dealt with, not yet satisfied. It is used about things people need or want or ask for — needs, demand, expectations, targets, goals. If a need is unmet, it is still there, still waiting, and nobody has taken care of it yet. Notice that we almost always put it BEFORE a noun: 'unmet needs', 'unmet demand', 'unmet medical need'. English speakers rarely say 'the need was unmet' after the verb — that sounds odd; they would say 'the need was not met'. The word is quite formal, and you meet it most often in reports, news, business writing, and medicine. 'Unmet medical need' is almost a fixed phrase in the drug industry: it means a disease that has no good treatment yet, which is why companies say it to explain why they are making a new drug. There is a quiet feeling inside the word that the need SHOULD have been met — someone was supposed to take care of it. So 'unmet needs' often carries a soft complaint, even when the writer sounds neutral. In an office you may also hear 'unmet targets', meaning the number people promised to reach and did not.