What is the meaning of the term 'reserve acidity'?
Reserve acidity, more commonly known as Buffer capacity, is the amount of acid/base (in gram equivalents) that must be added to a buffer solution to change it's pH by one unit.

In your question, the $\ce{CH3COO-}$ is able to react with any Hydrogen ions that are added to the solution, decreasing the $[\ce{CH3COO-}]$ while increasing the $[\ce{CH3COOH}]$. It takes care of any excess acidity, hence, it is called as the reserve acidity. Similarly, the $\ce{CH3COOH}$ should be called the reserve basicity, as it can reach with any base to prevent much pH change.

For a mathematical approach, we use the Henderson-Hasselbach equation for calculating the final pH.