OSINT is data collected from publicly available sources to be used in an intelligence context. If an attacker were to run a target phishing campaign (which is sending fraudulent emails pretending to be from a reputable company, in order to have them reveal personal information or click on a malicious link), it looks more credible if you have prior knowledge about the individual being targeted.
Its amazing at how much information people share about themselves on social media platforms, both intentionally and un-intentionally. The OSINT framework is https://osintframework.com/ is a collection of resources and tools you can use for your intelligence gathering.