biological filtration refers to the use of large surface area media to provide a medium for beneficial bacteria to colonize, and to accelerate the nitrification process. It is commonly called bio media, bio bale, or something bio in the fish keeping community.
Now I will admit that I am mostly using this from an aqua culture/ponic point of view, namely raising fish or plants or some combination of the two. I am not entirely clear on what role nitrification plays in the chemical interactions that occur over time in your rez.. however I do know that the presence of beneficial bacteria has a direct positive impact on the bacteria that is clinging to the roots of your plants, and its instrumental in preventing slime out breaks. To that matter, its what solved my dwc root slime issues.
This is an issue that has been discussed in depth in other threads, the only one I was able to quickly locate I have linked.
https://www.icmag.com/ic/showthread.php?t=254482
them ladies have their own tunes..