Metal appears as a bright area on an x-ray, blocking visibility of underlying structures. The reason you're asked to remove metal is to give the radiologist an unobstructed view of the area of interest. Basically, you remove metal because it blocks anatomy. If you have a metal implant, obviously you can't remove it for an x-ray, but if the technician is aware of it, he may position you differently to get the best imaging results or take x-rays from multiple angles.
The reason metal appears bright on the x-ray image is that it is extremely dense, so x radiation does not penetrate it as well as it does soft tissues. This is also why bones appear bright on an x-ray. Bones are denser than blood, cartilage, or soft organs.
The Issue of Metal in the X-Ray Room
Unless the metal item is directly in the path between the x-ray collimator and the image receptor, there's no issue having metal objects in the same room as an x-ray machine. On the other hand, metal objects are not permitted in a room housing magnetic resonance imaging (MRI) equipment because the objects will be drawn toward the powerful magnets when the machine is turned on. Then, the problem isn't with the image. It's a matter of the items because of hazardous projectiles, which could injure people or damage equipment.