It's kinda hard to answer succinctly, I made the comment after reading about how it works, here: https://github.com/lnbook/lnbook/blob/develop/03_how_ln_works.asciidoc

It's essentially a mixture of cryptography and game theory, that allows you to transact off-chain without fully trusting the people you're transacting with.

Almost like how escrow works, but without actually having to trust an escrow service.