Solid (derived from "social linked data") is a proposed set of conventions and tools for building decentralized social applications based on Linked Data principles. Solid is modular and extensible and it relies as much as possible on existing W3C standards and protocols.
What is the centralization that decentralized Web advocates are reacting against? Clearly, it is the domination of the Web by the FANG (Facebook, Amazon, Netflix, Google) and a few other large companies such as the cable oligopoly. These companies came to dominate the Web for economic not technological reasons.