The word fence has two main meanings:

1. A barrier made of wood, metal, or wire that marks boundaries between properties or keeps animals contained. Your backyard might have a wooden fence separating it from your neighbor's yard, or a farm might use a wire fence to keep horses from wandering off. Fences can be decorative, like a white picket fence in front of a house, or purely functional, like the tall chain-link fences around baseball fields.
2. To fight with swords as a sport. In fencing, two competitors wear protective gear and try to score points by touching their opponent with their weapon. Olympic fencers use specialized swords like foils, épées, or sabres. The sport requires quick reflexes, strategy, and footwork, almost like physical chess. Fencing has been practiced for centuries, originally as training for real sword combat, and it became an Olympic sport in 1896.

The phrase sitting on the fence means refusing to choose between two sides of an argument or decision, staying neutral when others want you to pick a position.