Common sense says price stability should be zero inflation and zero deflation. A dollar five years from now should have the same purchasing power as a dollar today. Of course, this purchasing power would be "on average," since some items are always going up or down in price for reasons that have nothing to do with the Fed.
And how you construct the price index matters also. It's an inexact science, but zero inflation seems like the right target. But the Fed target is 2%, not zero. If that sounds low, it's not.
Inflation of 2% cuts the purchasing power of a dollar in half in 35 years and in half again in another 35 years. That means in an average lifetime of 70 years, 2% will cause the dollar to 75% of its purchasing power! Just 3% inflation will cut the purchasing power of a dollar by almost 90% in the same average lifetime.