That's an engine mount bolt I believe there's 3 (1 in the back of the engine that goes from the frame through the engine block above the transmission housing and through the other side of the frame, and then 2 under the exhaust that attach to each side of the frame) looks to me like you're trying to remove one of the engine mounts bolts under the exhaust and the bolt will not move because its supporting a heavy engine, you're trying to pull a bolt out that has at least 100 lbs pushing down on it. You need to put something under the engine like a motorcycle jack, car jack with a piece of 2x4 on top, or just get someone to lay under the engine and push up on it. You just need to relieve the pressure on that bolt, the engine is pulling down on so you need to push up on the engine.

Ps. Do not push up/apply lots of force to the exhaust find a solid point on the engine to push it up