Underprivileged describes people who have much less than most people in their society — less money, worse housing, worse schools, fewer chances in life. It is not only about being poor at this moment; it points at the chances you never got, which is why it attaches so often to children and to schools and neighbourhoods: underprivileged kids, underprivileged areas. It is a careful, polite word, used by teachers, charities, news reports and governments — chosen exactly because 'poor' can feel blunt or shaming. You will also see 'the underprivileged' used as a group noun: "a charity that works with the underprivileged." Though actually, this word is going quietly out of fashion among people who work in this field. Their argument is that 'underprivileged' quietly treats a comfortable life as the normal baseline, and it describes the person rather than what was done to them; so you now hear 'low-income families', 'disadvantaged', or 'under-served communities' instead. It is not a rude word and nobody will be offended if you use it — but it is a word of the last generation more than this one, and if you are writing something serious, naming the actual situation ('families on low incomes') is usually stronger.