and why is it important?
First you need to understand what inflation is. A basic principle of economics is that goods and services are scarce, meaning limited quantities. Normally goods are exchanged as money, and historically that has been a rather wide variety of goods: cattle, base metal coins, precious metals in various forms, stones, beads, and more. Markets have occasionally used some form of markers as money to ease transactions.
Then comes "reserve" banking, which is a system that avoids reserves. The money is simply created, and the system is made as complicated as possible to discourage casual inspection. The government protects the bankers from threats and forces citizens to accept the money. The money is good as long as the government says it is good. When the government collapses, the money becomes trash. Like this:
(Bear in mind that at one time each of those Soviet paper rubles was worth almost as much as a US paper dollar, which was almost one ounce of silver.)
As long as the amount of paper money corresponds to the sum of all business dealings, the economy is stable. But that never happens. The bankers create all the money the government wants to spend, and the government likes that arrangement just fine. So the bankers create more and more money, and that is called inflation. When the money is honest there is no inflation and no unemployment. None zip zero nada goose egg.
So it becomes obvious that nobody in the system can ever be honest about it. The government maintains an office to monitor inflation, but the staff has clear instructions to never report anything bad. They fudge the numbers, or they report so many inane things that all reports are rendered nonsense. In USA the budget office reports consumer price indexes without housing, without transportation, without food, without anything you like, and each of those numbers is the official cost of living index.
You can learn a lot by reading the free materials at mises.org and fee.org