Due to their mass, objects like neutron stars or black holes tend to steal matter from their neighbourhood. In this case, we call that object an "accretor". Matter, plasma or dust is gravitationally attracted by those objects, but due to its speed, it can not fall into the black hole directly but forms circles around the  accretor. Driven by friction, vicious and magnetic force, the matter drifts inwards, which increases the speed, which can reach velocities close to the speed of light in most extreme cases. The friction process heats the matter, which results in an emission of thermal light, which can be as energetic as X-rays. This disk-like structure emitting thermal light is called an accretion disk.

Note that other objects can also lead to accretion disk, like a forming planet or star.