A blockchain is fundamentally a database, distributed over a certain number of nodes, constantly executing a "consensus mechanism" to decide which transactions should be added to that database, also called "Ledger". While the nodes of a certain blockchain are all connected to each other, two different bockchains do not have any nodes in common: in general, a blockchain is a network in its own right, isolated from all the other blockchains and, as a consequence, normally it is not possible to transfer tokens from one blockchain to another blockchain.
Cross-Chain Bridges are a relatively new technology in the Blockchain space. A cross-chain bridge allows transferring of value and any arbitrary data from one blockchain to another. It is usually deployed as a set of smart contracts on two or more blockchains, and a set of off-chain services that monitor/trigger these contracts to lock/burn/mint tokens and data on the respective chains depending on the type of the transfer.
Cross-chain bridges can be more or less decentralized, varying from totally centralized architectures, where users must trust one or a few intermediaries that control the smart contracts and the off-chain services, to fully decentralized bridges, where the smart contracts act fully autonomously and without owners, and anyone can join the off-chain services network and participate in the bridge consensus with their own node.
ioTube is a cross-chain bridge designed to be multi-chain and multi-asset, developed by the IoTeX team to seamlessly connect IoTeX to the popular blockchains like Ethereum, Binance Smart Chain, Polygon, and more.