A crossover is the point at which sound moves from one speaker range to another. Setting it thoughtfully can help the system sound clearer and keep speakers from working outside the range they handle best.
Start by understanding the frequency range of your speakers. Listen to a familiar track with bass, mid-range and high notes, then make careful adjustments rather than turning everything up at once.
If you are not sure where to begin, an installer can tune a system around the equipment and vehicle so the result feels balanced rather than simply loud.