The crossover frequency is the number of Hz that your subwoofer is able to reproduce at a given volume. Generally, the crossover frequency for a subwoofer is between 40 Hz and 160-200 Hz. But some satellite speakers cannot reproduce very low frequencies, so you may need to experiment a little with your subwoofer's frequency before deciding on a final setting. You should always check the manufacturer's recommended crossover frequency range before making any adjustments.
If you have a mediocre sound system, you can still set the crossover frequency to get a decent sound. You can also adjust the crossover frequency using a phase switch. This allows you to listen to the speaker without installing it fully. The best crossover frequency is 80 Hz. You can tweak this setting by bringing up the crossover control a little clockwise until you find a frequency that makes the sound you want.