To underrate something is to underestimate its value. If critics underrate a great movie, they don't give it enough credit for its brilliant acting and well-written script.
If most people underrate the Polish restaurant in your neighborhood, they'll be pleasantly surprised once they give it a try. High school football players tend to underrate the importance of gracefulness in their sport, and your math teacher may underrate the amount of time it takes you to do your calculus homework. The verb underrate has been around since the 1640's, although back then it was generally spelled with a hyphen: under-rate.