Hive is not programmable money. So, in order to use it as programmable money you have to get it onto the smart contract platform. So, you send Hive and you receive 1 swap.hive. When you're done you can return your swap.hive and get back your Hive. Hive-engine.com is the smart contract platform and you use the deposit/withdrawal buttons to move it on and off the platform.

Swap.Hive can be used for trustless transactions because the program can force it to go through. Otherwise one person has to send one thing and the other sends the other thing afterwards, which takes trust. Here you're trusting Hive-Engine, a business, to not steal the Hive you have on deposit there.