To undervalue something is to treat it as worth less than it really is. There are two quite different lives this word leads. The first is about money and price: a house, a painting, a company, a currency can be undervalued, meaning the price on it is below what it is truly worth. Business people and news reports use it this way a lot — "the shares are undervalued" is an invitation to buy. The second is about people and their qualities, and this is the one that stings. If you undervalue a colleague, you do not see how much they contribute; if a company undervalues its staff, people feel unseen and leave. You can also undervalue a thing that is not a person at all — sleep, practice, kindness, a simple idea — meaning you do not give it the weight it deserves. Notice it is nearly always about a wrong judgement, not a wrong feeling: you are making a mistake, and the word quietly says so. It is a bit formal. In everyday speech people are more likely to say "they don't appreciate her" or "we didn't realise how good it was". Also worth knowing: the passive is extremely common — "I feel undervalued at work" is one of the most-used phrases in the whole area of jobs and management, and "undervalued" there is close to "not appreciated, not paid enough, not thanked". Though actually, English is a little uneven here: the opposite, "overvalue", is used for money all the time but sounds odd about people — for people we usually say "overrate".