Cryptocurrencies like Bitcoin and Etherium are digital currency's designed to function as a medium of exchange. They are secured and verified by cryptography, as well as governed by a distributed network of computers. Blockchain technology is used by many cryptocurrencies to maintain a distributed ledger amongst these computers. Because they are not issued by any central authority, cryptocurrencies are not subject to government interference or manipulation. This is why you will hear the term "decentaralized" so much in Crypto.
Decentralized networks distribute control and decision-making from a centralized entity (like the government) to a distributed network. In blockchain, decentralization refers to the lack or no control from a centralized entity to a decentralized network. Decentralized networks attempt to minimise the amount of trust that participants must place in each other, as well as the ability to control or govern one another in ways that degrade the network's functionality.