Magnitude: Magnitude is how we measure brightness of an object. The measurement is old and based on the brightness of stars seen by the naked eyes of people 2,000 years ago. The main idea is that the higher the number the fainter the star, and the lower the number (also, usually a negative number) the brighter the star. For instance, Arcturus is magnitude 0, while the sun is magnitude -27. So there’s that.
Radiant: The radiant is a point in the sky from which a meteor shower appears to originate. A meteor shower is named for the constellation in which the radiant is located. For instance, the Leonid meteor shower has a radiant in the constellation of Leo.