What exactly is Threat Intelligence and how to collect it?

Hi I am trying to understand when someone says Threat Intelligence what exactly it means? There are multiple companies who are doing business in this area and I don't know what exactly their product is and what they sell. Does Threat Intelligence mean providing information about some attack before hands? And I am interested in knowing how threat intelligence is collected, what are the different sources and ways by which threat intelligence can be created/collected. Cheer's