This is a term used to describe malicious actors who perpetrate cybercrime.
Colloquially they are sometimes referred to as hackers or cyber criminals.
Threat Actors can include:
Foreign states
Criminal groups
Individuals
Many threat actors are well-organized groups who work together to perpetrate
sophisticated cybercrime. Their work settings can be very similar to your
own. Threat actors also work together with other groups to amplify their
work.
While their specific motivations, tactics, and techniques may vary, their
intention is the same: to exploit human and system vulnerabilities to
achieve their goals.