Price stability is when the inflation rate is low and does not fluctuate in the extreme. Stability does not mean inflation is zero percent. Say, the percentage moves between 1-2%.
A stable low inflation rate enables more accurate long-term business and consumer decisions. Inflation expectations will also be quite stable and from day to day. Say, you borrow money to the bank. Banks often charge interest that adjusts for inflation. If inflation moves stable, your loan interest percentage will not change much compared to before.