What that means in plain English is this. To the Rambam the most lenient is things learned from the 13 principles of interpretation. [For that a later court can reverse the decision even if they are just a small court.] The more strict level is decrees of the sages. For that a later court can reverse the decree if it is greater in number an wisdom. The most strict level is decrees made as a fence to Torah. That can never be reversed. That is A-B-C. The Raavad's system starts with the last thing being the most lenient. He says that that R.Yochanan Ben Zachai reversed the decree to bring the fruit fruits to Jerusalem and not redeem them. The next level he explains in his comments on tracate Eduyot where there is one of the 13 principles of interpretation that a later court can reverse if it is greater in wisdom and number. The Raavad there says the later court would not reverse the decree unless the minority opinion was recorded. It comes out then that the later court can go against that majority because it is a greater majority than the original court that ruled against the minority opinion.
Then the most strict is what is to the Rambam the middle level. That is decrees. There the Raavad says if it is נתפשט Then it can never be nullified.
Now you could say the Raavad is not disagreeing with the Rambam's division. But there is good reason to think that the Raavad is making the difference between halacha 2 and halacha 3 to be dependent on whether the decree has been accepted, not whether it is a fence to the Torah. You could argue this point but for the time being let's just say that that is how the Lechem Mishna and Rav Shach both understand the Raavad. That means that the Raavad is being strict in halacha 2 because as he says the decree was accepted in all Israel. That is why even a later court can not change the decree. And that means that in halacha 3 where the Raavad is the most lenient that is because the decree was no longer accepted. I mean to say that certainly the decree was once accepted. But when R. Yochana came around an the Temple had been destroyed it no longer was the custom to bring first fruits to Jerusalem. So he nullified the decree though he was smaller in wisdom and in number.
Not only that but it would seem like the Rambam would have to agree that once the decree was no longer accepted, it was no longer in force. For to the Rambam how was it possible for R Yochanan to nullify a decree when he was smaller? It was not a case of the 13 principles which is the only case the Rambam would have allowed such a thing.
That means that the Raavad and perhaps the Rambam also are thinking that decrees have force only in so far as they are accepted throughout all Israel. Once they are ignored they no longer have validity because the whole reason for their existence is gone.
So what comes out from all this is significant. That is that the Raavad is thinking slightly different from Tosphot. What Tosphot hold is numerous places is that if the reason for a decree is nullified the decree itself is nullified. [This is like Raba in Beitza page 5a] What you see from the Raavad is that what is determinant if if the decree is presently accepted throughout all Israel. Not if it once was accepted. R. Yochanan certainly did not reverse a decree that had never been accepted. Or which was based on teh 13 principles. Rather it was a decree that had once been accepted and then was ignored Thus R Yochanan could nullify it even though he was smaller in number and wisdom
To me this makes sense. I thought at first that if people would be more strict about Torah that that would make things better. But eventually I began to notice there was an inverse relation between strictness and menchlichkeit.