Its basically a layer of STEEM ENGINE, where tokens are traded against pegged token of STEEM.
Once you deposit, your STEEMP(pegged token) gets deposited into your own account controlled by your own keys, which you can access from any interface like steem engine, dlike dex etc and can withdraw which means convert back to STEEM! SO basically its conversion from simple steem to trade-able steem pegged token and fees are charged for conversion and your funds always remain under your keys!