OSINT stands for Open Source Intelligence. It is a comprehensive methodology and set of tools and techniques to gather & analyze information that a person or a company revealed about themselves while using Internet, smartphone apps and basically any digitalized asset or a service. And you know pretty well for yourself that modern world is vastly digitalized.
Don't think for a second that open source & freely accessible information is something useless or of no interest. People reveal a tremendous amount of personal data these days:
And many more. And it's all there, in that huge pool of data we call modern Internet. One just need to apply certain methodology, skillset and tools to obtain, filter, sort & analyze the data to compose a comprehensive digital portrait of a person or a company.