A' setting right the Scottish scores =?

A score, in this case, means vengeance. It is most often seen in the expression "i've got a score to settle with him." It comes from the custom of keeping score in a game by cutting a notch in a stick, and from that was made into a figure of speech. Late in the 13th century, King Edward of England invaded Scotland, and put it under the control of his trusted followers, who, so said the Scots, abused the people. In 1297, a man named William Wallace lead a rebellion against the English. He was unexpectedly successful, until defeated in 1304 at Falkirk. He was betrayed by a Scotsman, and turned over to King Edward, who had him tortured and executed.

One young man in that era was Robert the Bruce, of mixed Scots, Norman and French descent. About ten years after the battle of Falkirk, Robert the Bruce defeated the English at the battle of Bannockburn (a bannock is a baked oat cake, and a burn is a small stream--so it means the stream of the oat cake). He made Scotland independent, and he made himself King.

So, this poem celebrates Robert the Bruce, and his victory over the English, which was also seen as getting revenge against the English. Robert the Bruce is a Scots national hero to this day.

That makes little sense to me--but it may be an "unstandard" spelling of serf, or a transcription error. In that case, it would mean that without the men who fought with Robert the Bruce at Bannockburn, Scots would be serfs today. If you don't know what serf means, please look it up, i'm not up to explaining it right now.

If that's what you got by looking it up, then it was a very imprecise definition. The reason that European feudalism and "feudalism" in China or Japan are not the same (and there never really was feudalism in those two countries) is that serfs in Europe had rights in property, for as much as one might allege they were oppressed. Although they had to pay the heriot, the "entry fee" to the land of their fathers, if they did so, then one third of the land they farmed was theirs to do with as they pleased. They were definitely not slaves. A serf who could amass sufficient capital to buy land ceased to be a serf, as did his wife and his children. I didn't want to go into this for just this reason--it's just too complex for a brief answer.