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Radiation is energy in transit in the form of high speed particles and electromagnetic waves.
Well, the high speed particles are parts of atoms that have been given extra energy, so that they are traveling away from where they were, giving them a chance to interact with things, like shielding and people. One of the main things about EM waves are that they really are pure energy and contain no mass as we normally think of it. Well, realize that radiation is energy in motion. So, what it will do, like anything in motion, is try to slow down. Sorta like when your hand gets hot from friction, radiation gives up energy to surrounding atoms as it travels through some material.