What is OSINT?
OSINT is Open Source INtelligence. If you are the parent of a daughter in dating age, you have likely performed OSIN and did not realize that there was a name for it. Did you use Google or use other sites to check into the poor unsuspecting guy? Did you drag up all kinds of dirt causing your daughter a major eye roll? Oh, perhaps that is just my wife and I. Fortunately, our daughter knew that this would be coming and warned her perspective suiter that it would be a thing.
Open source intelligence is the ability to research a target using free or publicly accessible tools. These tools may be sites that you use frequently, or more obscure that you did not even know were around. It all really depends on what you are hunting.
Perhaps you have a suspicious email that you want to look into more deeply. In this case I would jump onto MXToolbox.com to use several of their tools.