Off-chain means activity happens outside the blockchain itself. The result may later be settled on-chain, but the full process is not written directly into every block.
Examples of off-chain activity
Exchange order matching, private agreements, payment-channel updates, some wallet operations, and internal platform balances can be off-chain. Layer 2 systems may also process activity away from the main chain before settlement.
Why off-chain systems are used
Off-chain activity can reduce cost, increase speed, and keep some information private. The tradeoff is that users may need to trust a platform, operator, or settlement design.
Off-chain does not automatically mean unsafe, but users should understand what is recorded publicly and what depends on another system.