Let's imagine this in a easy and understandable language. As a example; Imagine that your brain signals that leg to move. But the path to that leg the signal is going through is blocked. So that leg is paralyzed.
Hope this helps! Please don't ask for a diagnosis or medical advice here. This is only for informational purposes since i'm not a doctor. Gonna use this tag: #Bahealthi when making projects about health like this.