Low pass filter is the frequency at which the subwoofer will start being attenuated. E.g. if you use 63Hz, then subwoofer signal at 63Hz will be -3dB at 63Hz, and -XdB at an octave above (the crossover frequency times 2 = 126Hz). The X is the slope setting. It is set in the "network/subwoofer 3" section.
These settings are primarily for integrating your subwoofer with the front speakers. There are similar setting for the front speakers to set their high pass filter. How should you set these settings? Realistically, an average car audio speaker will sound sloppy when driven hard below 80Hz. At them same time, a subwoofer will sound sloppy (or no sound at all) at higher frequencies. 80Hz LP/HP both using 18dB slope is a good starting point. I personally prefer 63Hz as this results in a smoother, mellower, less offending bass specially if you have some recording that are badly recorded (e.g. kick drum sound exchagerated too much) setting crossovers at 80Hz should give more punch, but as I said it can sound more fatiguing with misbehaving records. Not every speaker can cope with 63Hz HP. 80Hz may be more prudent. In fact, if you use factory speakers or small 5 inch aftermarket woofers, 100Hz HP may be more appropriate.