A very simple explanation is:
Decentralized- no one person, or group of people, have control over the currency.
Centralized- one person, of group of people, have control over the currency.
Bitcoin, and all other POW (Proof Of Work (they have mining)) currencies are NOT decentralized. there are, of course some methods which would allow for a decentralized Cryptocurrency- I will not get into those now fo the sake of simplicity. However, the important thing to realize is that Bitcoin and the rest of the Cryptos are MUCH MORE DECENTRALIZED than Fiat! Fiat currencies are 100% centralized!
The way that it works with Bitcoin is: no one person has control over it, but mining farms (if you haven't heard this term, they are huge operations consisting of many miners which handle the transactions of Bitcoin on the Blockchain) have a loose grasp on the centralization of Bitcoin. In other words, if a mining farm decided to stop mining, it would a have a big effect on the Bitcoin network, but the hole it creates could be easily fixed. Fiats are controlled by CENTRAL Banks- completely and utterly- meaning that if the Central Bank makes a decision, they immediately and wholly change the currency and that decision is final and unfixable by the people. There is no way for anyone to have such a grasp on Cryptocurrencies as Central Banks do on Fiat.
Good luck and I hope I explained this well enough!