A bulb in photography is basically a setting on your camera that allows you to control how long the shutter stays open when you take a picture. Imagine your camera is like a big eye that opens and closes to take a photo. Normally, the eye quickly opens and closes, like a quick blink, to capture the image. But with the bulb setting, you can keep the eye open for as long as you want, like when you stare at something for a long time without blinking.
Why would you want to do this? Well, sometimes there are really cool things to capture in a photo that happen over a long period of time, like fireworks or starry skies. But since the eye blinks so quickly, it can't catch all of those cool things in just one quick snap. By using the bulb setting, the eye stays open for longer and can capture all of those cool things happening over time in just one photo.
To use the bulb setting on your camera, you usually have to hold down the button that takes the photo for as long as you want the eye to stay open. This means you have to keep your camera really still to avoid making the photo blurry. But with some practice and a tripod to hold the camera steady, you can take some really amazing photos with the bulb setting!