A low pass filter is just that. It cuts off high frequencies that are perceived as sharpness / contrast at the expense of causing moire patterns. Some low pass filtering is also usually done in firmware on conversion to jpeg.
This has been known for a very long time as the Nyquist limit. As we get smaller and smaller photosites, the Nyquist limit is increased, and the possibility of having a usable camera is increased as in the Nikon D800. Even then, it needs filtering in software / firmware to soften the image and reduce moire.
However my 5D MK III is not soft. Being limited to studio work where lighting can be controlled to minimize moire patterns is the penalty for removing the filter.