This song gets a bad name as being anti-English - it's NOT. It's pro-Scottish. It is most definitly about the battle of Bannockburn, which took place in 1314.

Edward the Second came across the border with a huge English army, and was met by Robert The Bruce and the Scottish army, just outside the town of Stirling. The ground was very boggy, which didn't suit the English with their heavy horses. The Scots were outnumbers 3-4 to one, but because of the ground being so well chosen, the English couldn't bring the entire mass of their army to bear against the Scots. The Scots won, and routed the English. The victory finally re-established Scots independence, although it was quite a few years before the English acknowleged the fact.

The meaning of the song is that these men were the flower of Scotland, and established Scottish independence, and as the lyrics say, "When will we see their like again?" It's disliked by the Unionist parties because of that message, so they try to piss on it, but the people of Scotland, generally speaking, love it.