What does on-chain mean?
On-chain means the transaction lands on the Bitcoin blockchain itself. You broadcast it, miners include it in a block, and the network confirms it. From then on it is part of the permanent ledger that every node keeps a copy of. Nothing else has to happen and no other party has to cooperate: the chain has settled it, and that is final.
The contrast is with Lightning, the fast layer built on top of Bitcoin. Lightning moves small payments in seconds by keeping them off the chain until it matters, while on-chain is the base layer underneath where the real settlement happens. Both are Bitcoin. They just work at different speeds for different jobs.
When do you want on-chain versus Lightning?
The trade is speed and cost against finality and independence. On-chain transactions take minutes to confirm and carry a network fee that rises and falls with how busy the chain is. In exchange they are completely self-contained: no channel, no counterparty that needs to be online, just a settled entry on the ledger. That makes on-chain the right tool for larger transfers, for moving funds into cold storage, and for opening or closing the Lightning channels themselves.
Lightning is the opposite trade, and it is why you do not pay an on-chain fee every time you leave a small tip. The everyday small stuff rides Lightning; the larger, settle-it-for-good stuff goes on-chain. A typical sovereign setup uses both, each for the part of the job it fits.