The word bolt has several meanings:

1. A metal fastener with a threaded shaft and a flat head. A bolt works with a nut to hold things together tightly. Construction workers use bolts to connect steel beams in skyscrapers. Mechanics use bolts to assemble engines. Unlike screws, which dig into material as they turn, bolts pass through holes and get secured by nuts on the other side. When something is “bolted down,” it's fastened so securely it won't budge.
2. To run away suddenly, usually from fear or panic. A startled rabbit might bolt when it hears a loud noise. A nervous horse might bolt if something spooks it. When students bolt from the classroom when the bell rings, they're rushing out quickly (though hopefully not from fear!). The word suggests sudden, rapid movement without warning.
3. A flash of lightning. You might see a bolt of lightning streak across the sky during a thunderstorm. The phrase “a bolt from the blue” means something completely unexpected, like lightning from a clear sky.
4. A sliding metal bar that locks a door. You might bolt a door shut by sliding a metal bar across it for extra security. Castles had heavy bolts on their doors to keep intruders out.