A tune-up is a procedure that's done periodically to keep a car running well. You might want to take your car in for a tune-up before leaving on a cross-country road trip.
A tune-up is typically a series of small repairs and adjustments to a car's engine — not so much to fix something that's broken but to help the car run more efficiently or to keep it running longer. Sometimes tune-up is also used in the context of sports or exercise: "The volleyball team had one last tune-up before the big match." Tune-up comes from the "musical adjustment" meaning of tune.