Open Source Intelligence is the operation of gathering information about target, from publicly available sources. You don't use anything illegal, you don't get into contact with target so you don't leave trace. In short, you don't do anything to alert the target.
When harmless/unimportant looking publicly available informations about target systematically collected and gathered together, it becomes harmful, especially in social engineering attacks.
Examples of resources for OSINT:
Examples of tools for OSINT:
- Google Dorks: Sophisticated Google searches, shortly.
- Shodan: Search engine for inter-connected devices
- EXIF-Viewers: Sometimes you can find very useful information inside photographs
- Metagoofil: Info gathering tool for extracting metadata from public sources.
And many others...