Imagine driving on the freeway in a (partially) self-driving car like a Tesla. While you drive the car, you come across things you would expect like trees, lampposts and other cars but also things that don't belong there like trash floating around. Meanwhile, radars and sensors in the car are working hard to make sure you don't crash because of these things. If you see the freeway as your fast-changing IT environment, then all the things that don't belong there are anomalies.