Fred is the Mystery Inc. member who drives the Mystery Machine.
Fred Jones is the gang's de facto leader and is normally shown behind the wheel of the brightly colored van. The Mystery Machine carries Fred, Daphne, Velma, Shaggy, and Scooby-Doo from one case to another, making it nearly as recognizable as the characters themselves. Fred's role as driver fits his practical, organized personality and his leadership within the group.
Fred is also associated with designing elaborate traps for villains. These Rube Goldberg-style contraptions frequently go wrong when Shaggy or Scooby accidentally triggers them, but the gang usually captures the costumed criminal anyway. Fred commonly investigates with Daphne, while Velma, Shaggy, and Scooby pursue other clues.
Other characters may drive the Mystery Machine in particular episodes, films, or alternate continuities, so Fred is not literally the only person ever shown at the wheel. Nevertheless, “normally drives” is the standard franchise description, making Fred the canonical answer. The original animated series premiered in 1969, and Fred has been primarily voiced by Frank Welker since the character's introduction.