The Pixar film Cars introduced Mater before he appeared in later Cars stories.
Mater is Radiator Springs' rusty tow truck and becomes Lightning McQueen's closest friend. He first meets McQueen after the race car is sentenced to repair the town's damaged road. Mater's friendliness, unusual stories, and enthusiasm for tractors quickly contrast with McQueen's initially self-centered personality.
Larry the Cable Guy voices Mater, whose full name is commonly given as Tow Mater. The character became popular enough to lead Cars-related shorts and to play a major role in Cars 2, where his accidental involvement in an international spy plot drives much of the comedy.
Mater's Tall Tales is a separate animated series of short segments, not the feature film that first introduced him. Mater also appears in Cars 3, but that later appearance cannot be the answer to an introduction question because the character was already established in the 2006 original.