Basically, taking the moral high ground means you achieve a point in which you can justify your own actions over the actions of your rival/opponent/fellow debater. This is generally followed by patronizing over him and so on (When mis-used, of course), taking the moral high ground in a war means the reason
you go to war is just and that you "Fight for the forces of good" against the "Force of evil".
The perfect example for this is the Second World War. The "Forces of evil" - Also known as the Axis states, started a war with the allied nations, or the "Forces of good", Japan attacking Pearl Harbor and Germany attempting to conquer all of Europe, North Africa and eventually the world. The main reason the Allied nations went to war was to stop this "Evil" from occurring and I honestly remember some leader claiming the Axis nations are the forces of evil, I just cannot remember which one or when.
Generally, the Allied nations fought for their own survival and to save the handicap, those with genetic defects, mentally ill/retarded, Jews, Gypsies and many other peoples (I apologize if I had forgotten any) from being destroyed by Nazi Germany.
Fighting for survival and for the greater good along with saving many other peoples from death and destruction gave the Allied states the moral high ground and made them the "Force of good". Generally, in cartoons, if you see a superhero fight against a villain, notice that the superhero
always has the moral high ground. Be it stopping a murderer, saving hostages and so on.
Now, in debates you can use this tactic to show that, morally, your argument is more valid/true than that of your opponent in debate. This can also be explained by a
moral conscience.
In my eyes, the best way to explain the moral high ground is to make sure others abide by the law and fight
for the law. For example:
Every man has the freedom of speech. If you see said freedom being infringed, you are obligated to report it to the proper authorities or bring it to light - You also have the moral high ground in a debate over that predicament, as the basic rights of the human being were taken away from him. From that you can also see that the moral high ground can come from keeping and abiding by the laws/rules of society and/or your government.
That's my explanation of the moral high ground. Hope it helps.