Accretion describes gas and dust spiraling into a black hole and releasing energy.
As material falls inward, it usually forms a rapidly rotating accretion disk. Friction and turbulence heat the disk to millions of degrees, allowing it to shine strongly in visible, ultraviolet, or X-ray light. The black hole itself remains dark, because light cannot escape from within its event horizon.
Accretion is not the same as the black hole swallowing everything nearby. Material must lose orbital energy and angular momentum before it can move inward. Some matter may instead be expelled in powerful winds or narrow relativistic jets.
A common mix-up is confusing an accretion disk with the event horizon. The disk is hot, visible material outside the black hole; the event horizon is the one-way boundary surrounding the interior.