In the simplest possible terms, cyber threat intelligence (CTI) is the process of taking disparate pieces of information about a cyber attack and identifying the context in which it happened and what that means. It is the same process a traditional intelligence analyst working for the military or law enforcement would apply to an investigation; it just sits within the realms of cybersecurity.
The earliest example of CTI, at least in terms of prominent media coverage, is probably the Mandiant APT1 ...