Engine is a metachain on top of Steem. What that means is that it is not running in the consensus layer of Steem but runs using the Steem blockchain as a base. Here’s steem-state which it uses to run. Go to ‘what is soft consensus’ and you will get a deeper explanation https://github.com/nicholas-2/steem-state
Its supposed to be able to run more complicated DApps than Steem currently supports (eg NFTs).
Right now to use it you have to install the node software and use the command line. Its quite complicated but heres a link.https://github.com/nicholas-2/engine-steem/wiki/Node-Owner's-Guide In the future a web interface will be created once the mainnet is launched.