Modified duration meaning

The formula for Mod Dur is -P’(i)/ P(i) where P(i) is the function of Price with respect to YTM. But I don’t get why it imply the percentage price change if market yield changes by 1%. Wouldn’t that be mathematically inappropriate? Can someone please explain it

Suppose that you have a bond that was selling at 980.25. Interest rates fall, and now it’s selling at 991.75. What’s the percentage price change? How did you calculate it?