If inflation isn’t based on most prices increasing… What is it based on?

It’s the devaluation of currency that happens when too much of it chases too few goods in the marketplace. It’s purely a monetary thing, you get that when the supply of money grows more quickly than the value of real goods in the economy does.

Ideally, we print money (and take it out of circulation) at a pace that keeps the money supply more or less balanced to the value of available goods and services in the economy. If we were to print too much money, or not take enough out of circulation (note: paying taxes does this; when you pay taxes the money doesn’t go into some account somewhere, it’s used to zero out the bonds issued to create it), the amount of money in circulation would become greater than the amount of real valuable goods in the economy. When that happens, the resulting bidding contest to secure those goods (after all, money doesn’t have intrinsic value, it’s only good for buying things that do) drives up the price of those goods in monetary terms.