Decentralized means that something is not in the hands of a central party. A decentralized process takes place without the involvement of an intermediary. Examples of central parties are companies, governments, and banks.

Instead of a central party deciding what to do, authority is spread over all connected persons or devices. Not one party is responsible for the ins and outs within a decentralized entity, but everything and everyone that is connected is responsible.

The US Dollar is managed centrally and Bitcoin is managed decentrally. Bitcoin is currently the only truly decentralized cryptocurrency.