Basically, he's THE horror writer of the Sixties and early Seventies. Before Stephen King, it was Matheson. His literary work covered novels, shorts, television screenplays, movie adaptations.
Matheson's greatest contribution was the updating of the horror setting from the 19th Century (or earlier) to the 20th. A lot of horror and fantasy by the 1960s had been stuck in the era of Edgar Allan Poe and Hawthorne, in some gothic nighttime landscape of ruined castles and decaying nations. Matheson brought the horror into the suburbs and cities: his I Am Legend not only redesigned the vampire genre, it basically launched the concept of a Zombie apocalypse (and redefined the concept of just who the monsters really are), and doing it in a world most readers would see as their own (amplifying the horror). His heroes no longer soldiers or priests or barons in pursuit of the supernatural: they were common blue-collar workers or office drones, suddenly confronted with a real-time terror that wouldn't go away with prayers to a distant God.
RIP, Mr. Matheson. ...Just don't mind if we, uh, shove a metal stake into your heart to make sure... WATCH OUT HE'S GETTING UP HE'S