Most experts agree this song is about the 1746 Battle of Culloden but there’s disagreement about who is singing and what they are doing. You can read about some of those at Wikipedia.
The way I learned it—from Grandma Swanstrom, I think—the singer is a woman who has lost her lover in the war. “Ye’ll take the high road, and I’ll take the low road, and I’ll be in Scotland afore ye.” Her love will be hanged (the gallows is the “high road”). She’ll return the ordinary way (the “low road”). And she’ll get there first because he’ll be dead and in heaven.
I didn’t even know there were other interpretations. Two that I’ve read about the past few days:
One, that it’s sung by the soldier to his lover. She’ll take the high road back, which is the normal way, and he being dead will take the fairy road (“low road”). He’ll get there first because his arrival will be instantaneous.
And two, that’s it’s sung by one of two soldiers. One will be released and the other will be executed. That theory also splits into two, whether the one who takes the high road is the one who is executed or the one who survives.
Fun to think about but I’m sticking with the way I learned it.