I'm not sure that's a real term, but there are nutrients companies out there that claim that their nutrients will stay at the correct pH without the need for adjustment. From what I've seen, that's true if you have a standard City water that has a low parts per million and i ph The Falls between 5 and 7. Those nutrients work really great. But as soon as you get out on well water or get some real alkaline water you run into problems.
So by buffer, I guess what I mean is hold the pH stable. Soil can do that naturally to an extent. But your water if you think about it has a whole bunch of something in it already. Your water could be all calcium that would have a different effect than if it was all iron. And nutrient companies can't make a nutrient line that will be able to buffer in every situation or hold stable in every situation.