Payment channels: two parties, one on-chain footprint
The Lightning Network is a layer built on top of Bitcoin that lets two parties transact an unlimited number of times while only touching the blockchain twice, once to open a channel and once to close it. To open a channel, both parties lock funds into a single on-chain transaction. From there, they can send funds back and forth between themselves by exchanging signed balance updates privately, off-chain, each one superseding the last. Only the final balance actually gets settled on the blockchain when the channel closes.
This works because each new balance update is cryptographically designed to invalidate the previous one, so if either party tried to broadcast an old, outdated balance to cheat the other, the honest party can prove it's outdated and claim the funds themselves as a penalty. That enforcement mechanism is what makes it safe to trust off-chain updates without needing a third party.