As others have already said, the low pass filter on a sub determines at what rate the sub's output vs. frequency rolls off. If the sub is set at 60Hz and has a 1st-order low pass filter, 120Hz will be reproduced at only 6dB down from 60Hz, 180Hz at 12dB down. In contrast, with a 4th-order filter and a setting of 60Hz, 6dB down is not reached until 240Hz.
The higher the crossover frequency chosen, the more steep should the low pass filter be (1st-order is 6dB/octave, 2nd-order 12dB, 3rd-order 18dB, 4th-order 24dB). And of course the lower the crossover frequency, the more shallow can be the low pass filter. Conversely, the steeper low pass filter, the higher can be the crossover filter. When I spoke with Roger Modjeski (Music Reference) about mating subs with the Quad ESL, he told me he used a 4th-order filter and a crossover frequency of 100Hz with his.