Mono Bass is exactly as the name suggests. It collapses your low frequencies to mono.
Having a nice wide stereo spread is great, expect for when we are talking about low frequencies. Mono Bass fixes the wide bass problem by collapsing your low frequencies to mono to prevent that odd, head-turning wide bass issue. Just set your frequency, and everything below that is brought to the center.
Most of the time the effect is quite subtle (or at least it should be if your mix isn't completely out of wack) but it definitely solidifies the low end and fixes a lot of the awkwardness of extreme panning on headphones.
Just for added measure, BDL added a width knob to the higher frequencies, making a great stereo widener without "weirdifying" the low end.