Bloom (sometimes referred to as light bloom or glow) is a computer graphics effect used in video games, demos and high dynamic range rendering (HDR) to reproduce an imaging artifact of real-world cameras. The effect produces fringes (or feathers) of light around very bright objects in an image, obscuring fine details. Basically, if an object has a light behind it, the light will look more realistic and will somewhat overlap the front of the object from the 3rd person perspective.
Performance Note: Because of the obvious change in graphics bloom could cause some computers FPS to drop. The drop may not be too severe to some computers or may not even effect it all together.