Active galactic nuclei (AGN) are extremely luminous and highly energetic sources of radiation that are found at the centers of some galaxies. They are powered by accretion of gas onto a supermassive black hole that is millions to billions of times more massive than the Sun. The high-energy processes occurring in AGN lead to the emission of various forms of radiation, including X-rays, gamma-rays, and radio waves. AGN activity refers to the processes and phenomena associated with the accretion of material onto the supermassive black hole at the center of the galaxy. This includes the formation of an accretion disk around the black hole, the generation of powerful jets of particles and radiation, and the production of intense electromagnetic radiation from the surrounding gas and dust. Studying AGN activity is important for understanding the growth and evolution of galaxies, as AGN are believed to play a significant role in shaping the properties of their host galaxies. Researchers use a variety of observational techniques, such as X-ray and radio telescopes, to study the properties and behavior of AGN in order to gain insights into the physics of these extreme objects and their impact on their surroundings.