What does bulb mean?

Sorry if this sounds stupid - I've had my Nikon D5000 for almost exactly a year now and I'm still learning the ropes. When shooting in manual I've noticed that the shutter speed goes to 30 seconds then goes to a Bulb setting. What does this mean?

Bulb means that when you press and hold the shutter button down that the shutter will stay open as long as you hold it. You can also use a remote shutter release to do this.

Bulb mode is used for any shutter speed longer than 30 seconds. As the answer above says, the shutter stays open as long as you hold the button. The best way to use it is with a wired remote, that you can lock. Use it for light trails on roads, star trails, or high speed photography, where you use a long shutter in the dark, and freeze action with a flash (the flash is faster than your shutter can go).

If you set your shutter speed on Bulb, it allows you to keep it open for as long as you need by holding the shutter release down. Not a stupid question.

In the old days we used a shutter release for large format cameras. It was a little length of hollow hose. One end attached to you camera shutter release and the other end had a squeeze ball or bulb. When you squeezed it air was forced down the tube and pushed out a plunger which triggered the shutter. Normally it would trigger at what ever speed your shutter was set at. If it was set at "B" then as long as you squeezed the ball the shutter remained open.
Same today but more modern releases.

30 seconds is the longest shutter speed you can set and still use the camera's self-timer to release the shutter.

The term "Bulb" dates back to the days when the shutter was pneumatically actuated. The photographer would squeeze an air bulb to open the shutter and the shutter stayed open until the air bulb was released. On your camera its the shutter setting that will allow you to keep the shutter open as long as you want. Instead of an air bulb you would need either the Nikon ML-L3 Wireless Remote Control or the Nikon MC-DC2 Remote Release Cord.

Since you say you're still learning about your D5000 this video tutorial is worth your time:
http://www.nikondigitutor.com/eng/d5000/index.shtml

Since you say you've been shooting in Manual it won't hurt to brush up on the Exposure Triangle.
http://www.digital-photograohy-school.com/learning-exposure-in-digital-photography