A price index is a composite of prices at a point in time. Sum the prices for each year. Then, select a base year, e.g., 1992. Divide the current year prices by the base year prices and multiply by 100. For any other year, the current year prices and the base year prices will most likely differ.
Price stability is measured by the annual percentage change in the price index.
Inflation is an increase in the general level of prices, as measured by some price index. Notice that not all prices rise during an inflation. Some prices may be falling, but those prices which are rising tend to pull up the general level of prices. Deflation is a decrease in the general level of prices, as measured by some price index. Notice that not all prices fall during an deflation.