Web3 or web 3.0 is the the concept where the internet moves towards a decentralized in ownership and influence. Web3 has often been associated with blockchain technology as one of the building blocks to achieve this new concept.
Web3 is the successor of web 1.0 (the inception of internet) and web 2.0 (internet services mostly centralized) and is still in the relatively early stages, read more about the differences here. Web3 is often mentioned as a synonym the metaverse or metaverse worlds, but web3 is more of a concept that is not tied to any specific execution of the concept such as the metaverse or blockchain technology.