To tune up is to balance or adjust a car's engine or a musical instrument so that it works correctly. You better tune up your instrument before a concert, or else you could turn off your fans.
Every motor vehicle needs a tune up now and then, and even your bicycle can benefit from a tune up. When a mechanic tunes up a car, she checks the systems and important parts and makes any needed adjustments. When a musician tunes up a guitar, he changes the instrument's pitch so that its sound isn't too sharp or flat. The musical meaning of tune up came first.