-Is just that, it lets all the high(er) freqs pass through and blocks out or (smuthers the lower frequency at the selected slop) 6,12,18,24db/oct etc
-Opposite, lets all the low(er) freqs pass and blcosk the high(er) freqs.
Depending if its a selectable filter or non.
For lows you basically want anything under 100hz or so to go to the subs (adjustable by taste and speaker setup)
For highs generally 100-120hz and up.