Someone underprivileged doesn't have the advantages other people have. Underprivileged people usually live in poverty.
A privilege is a right or an advantage, and people who are underprivileged lack such rights and advantages. Many times, this word is used as a synonym for poor. People often worry about underprivileged children who are living in poverty and may not have access to healthy food or good medical care. Underprivileged children often go to the worst schools too, which is another disadvantage.