This poem starts out with a highwayman (that's a robber who holds people up on the road) visiting his girlfriend Bess at her father's inn. He's on the move (apparently he's got some robbing to do) so he only has time for one kiss. He promises that he'll be back by the next night at the latest.
The next evening, instead of the highwayman, some British soldiers show up. These guys are real jerks. They drink a bunch of beer, they tie up Bess, and then they wait at the windows to shoot the highwayman when he comes back. Bess is tied up with a gun at her chest, and she wriggles around until she has her finger on the trigger. Then, when she hears the highwayman's horse, she fires the gun, and gives her life to warn him about the ambush.
The highwayman tries to get away, but it doesn't work. He gets mowed down by the soldiers in the middle of the road, and dies in a pool of blood. Still, on certain winter nights, his ghost still rides down the highway to meet Bess.