Mandotim has it nailed.
It's just a way of inserting an ancillary device. (Could be anything really, If you were mixing one of those X factor people, I'd recommend an Antares pitch shifter!) into the audio path of a channel.
Splitter leads should be marked send and return on the 2 Mono Jacks. If not, it won't cause any problems if you plug them in the wrong way round. It just won't work!
Swap them over, and all should be fine.
But don't worry, Your beloved H3000 FX box won't burst into flames. honest!