Undercooked food has probably not been heated through enough. For example, undercooked chicken has not been heated to a temperature that will kill bacteria like salmonella, which can make someone sick.
Not all undercooked foods are unsafe to eat. Some recipes call for undercooked rice or pasta that is more firm or even crunchy. Describing something as undercooked may be more of an opinion than a fact. Perhaps you think a perfectly cooked sweet potato is nice and firm, but your sister might call it undercooked because she likes it soft and fluffy. Non-food things might be undercooked as well — like a plan that is not well thought out or a team that isn't yet playing well together.