Short answer
The D7 visa is for non-EU citizens with passive income, such as pensions, rentals, or investments. You need a stable income of at least the Portuguese minimum wage per month, plus 50% for a spouse and 30% per child. You must also have accommodation and health insurance. The visa allows you to live in Portugal and, after renewals, apply for permanent residency or citizenship. Rules change, so check with the Portuguese consulate.
General information, not legal or tax advice. Rules and amounts change: confirm with official sources.