Subsonic filters (a.k.a 'high pass filter') remove information under a certain frequency (called a 'cutoff frequency'). The idea of this is if you have limited excursion or power, you can limit how deep the subwoofer goes.
The reason they mention vented enclosures is because below their tuned frequency, the driver tends to decouple from the box. Basically, this means that for the power your putting in and excursion your asking of the driver, your getting significantly diminished returns in output. To maximize your headroom, generally a subsonic filter is used to keep the subwoofer working where it is most efficient/designed to run.
I generally recommend everyone uses a subsonic filter. Every subwoofer reaches a point where it has diminishing returns, and as you go deeper in the frequency spectrum you really use up a lot of your headroom to get very little extra output. People love saying their system goes to 20Hz, but most of their systems would be much better for movies if they had a subsonic filter at 25Hz or so, with exception of some people who have monster bass setups.
