"On-chain" means the data lives on a public blockchain (e.g. who owns a #Name, who sent USDC). "Off-chain" means it lives in a normal app database (e.g. your portal description, your map pin, your Stripe receipts). Most of hashtag.org is off-chain for performance; the ownership-of-truth bits (your #Name, your USDC) are on-chain.
Web3 & crypto basics
On-chain / Off-chain
Recorded on a blockchain vs. handled in a private database.