Some types of energy can be seen or felt, such as visible light and infrared radiation. Some types cannot be detected without special equipment. Ionizing radiation cannot be seen or felt. It must be detected with special equipment. Ionizing radiation, unlike infrared, microwave, lasers, and most ultraviolet radiation, is energetic enough to remove electrons from their orbit about the nucleus of an atom.
Ionization changes the atom.
If the atom is part of a living cell, those changes could cause a health effect. You are probably familiar with x-ray radiation. X-rays pass through objects and expose film. This is why x-ray radiation is useful in many applications, from medicine to security to radiography of welds and other critical structures. X-rays are ionizing radiation.