The terms on-chain and off-chain describe where an activity or piece of data lives. On-chain means it is recorded directly on the blockchain, visible to everyone and secured by the network. Off-chain means it happens outside the blockchain, such as on a private server or a separate system.
The distinction matters because it affects transparency, cost, and speed. On-chain actions are permanent and verifiable by anyone, but they can be slower and cost a network fee. Off-chain actions can be faster and cheaper, but they do not carry the same built-in transparency and security.
Many systems combine both. Some record only essential information on-chain while handling the rest off-chain for efficiency. Understanding the difference helps you interpret how a project actually works and where its guarantees do and do not apply.