The math looks scary but it's mostly just a precise way of saying something you can understand in plain English. Once you get the concept, the equations are just bookkeeping. Focus on the idea first, the symbols second.

This is actually a much deeper question than it appears on the surface. The simple answer most textbooks give is technically correct but misses a lot of nuance. Let me try to explain it in a way that captures the full picture.

This is something I struggled with until a professor explained it using an analogy that finally clicked. The real world is messy and doesn't always fit neatly into the simplified models we learn in textbooks.