Much like a real world contract between - say - you and a bank when they accept your deposit, a smart contract should contain code provisions to revert the funds back to their owner if a certain set of circumstances happens in the environment (i.e. in the blockchain).

For example there could be, as part of the contract, much like there are exceptional provisions in real world contract, a M-of-N sig allowing the funds to be reverted back to the sender if enough (M) people agree that it's the correct thing to do.

This particular contract didn't seem to have any such provisions, which made it very unsafe.

Current smart contracts aren't very sophisticated yet, this is still the stone age for programmable money.

Things will improve, but in the meantime ... be safe out there.