Open-source intelligence, or OSINT for short, is the collection, analysis, and dissemination of publicly available information for intelligence purposes. OSINT may include information gathered from social media (SOCMINT), geospatial data (GEOINT), public records, and much more. A rule of thumb is that the information to be gathered must be publicly accessible without any restriction whatsoever. Various techniques and online tools are available to assist in gathering those information.
If you are not sure where to start, you may read this PDF file which I made for Jadi Hacker’s online class about OSINT. The material is by no means exhaustive – it’s merely a starting guide for those who are wholly unfamiliar with the concept.
If you would like to learn more about OSINT, I highly recommend you to read “OSINT Techniques: Resources for Uncovering Online Information” by Michael Bazzell. The book is very comprehensive and practically serves as a one stop shop for OSINT tools and techniques. Of course, you can always choose to research them by yourself.