In terms of Crypto currencies and Blochain technology, "centralized" refers to a structure when there is a central point or authority that controls the whole network/project.
A good example would be a Centralized Exchange, so an exchange that has an owner who controls the whole exchange. This owner has control over the funds deposited and whatever change they do to the exchange, will affect everyone involved.
Centralized setup is a Trust System as the users have to trust the managing authority, since they have full control.