One of the most important things about crypto, if not the most important, is the decentralization aspect. In the traditional financial world, everything is controlled by a central entity, there is a central database, central system etc ...
Crypto is different since it's using the blockchain, that is distributed over hundreds or thousands of locations, all carrying a copy of the same database (for example). There is no central place, where you could make changes to the blockchain, no new entry is possible without a consensus reached by the parties involved.
To put it in a simple way, decentralized means being distributed across multiple locations that work together in order to reach certain goal.