The idea of having a differing viewpoint our pointing out an error without having the intent thereof spill over into any other aspect of the interchange, and to take a forensic approach (where you open yourself to the possibility of being incorrect) as opposed to an argumentative one (where you maintain correctness regardless of facts at hand). Much in the same vein as "agree to disagree", but without implying the end of the exchange.
Okay, you want it in English? Let the X-man get to the 'splaining.
Disagreeing without being disagreeable means that you are allowed to not agree with the person you're talking, but your differing viewpoint do not reflect on the otherwise connection you have. It's an agreement to limit the scope of rebuttal to a clearly defined arena, e.g. the topic at hand, and not resorting to personal attacks and attempted character assassination.
Example of how not to accomplish this, taken from an actual (anonymous) Slashdot comment thread:
I can't believe how stupid you are. What an incredibly asinine position to take.
This is supremely bad because it gives nothing new, but instead is simply at attack on the person, erm, personally. This is the wrong way of doing it as it is generally assumed that the moment you resort to attacks outside the scope of the subject, you admit to not having a good rebuttal (as outlined in Basic Debate by Leslie Phillips).
The emphasis here is, as has been stated before, to have debate over a simple pissing contest. When you're having an argument, you are trying to prove you are right. In debate you are trying to come to a common absolute ground by an even exchange. Nothing was ever solved by people shouting at one another; just look at divorce.
The point of this sort of procatice is to keep the battle on the battlefield and to be able to look the other person in the eye without any sort of contempt after your discussion is concluded.