Socioeconomic describes anything that involves social conditions and money conditions together — because in real life the two cannot be pulled apart. Your family's income, your parents' jobs, how much schooling they had, the neighbourhood you grew up in: put all of that together and you have someone's socioeconomic background, or socioeconomic status. It is a research word, and it belongs to careful writing: government reports, surveys, education studies, health studies, news articles built on those studies. Common partners are 'socioeconomic status' (so common that researchers shorten it to SES), 'socioeconomic background', 'socioeconomic factors', 'socioeconomic group', 'socioeconomic inequality'. Why do people use it instead of just saying 'rich or poor'? Partly because it is more exact — it includes education and job type, not only money — and partly because it is politer. 'Children from poorer families do worse in exams' and 'children from lower socioeconomic backgrounds do worse in exams' say much the same thing, but the second sounds neutral and scientific. That politeness is worth seeing clearly: the word can soften a hard fact until the reader stops feeling it. Good writers know this and use it on purpose.