NOTE: master branch is for Steem Engine, for which further development has been discontinued. Only Hive Engine is currently under active development; This is now at its own repo: https://github.com/hive-engine/hivesmartcontracts.
Steem Smart Contracts is a sidechain powered by Steem, it allows you to perform actions on a decentralized database via the power of Smart Contracts.
This is actually pretty easy, you basically need a Steem account and that's it. To interact with the Smart Contracts you simply post a message on the Steem blockchain (formatted in a specific way), the message will then be catched by the sidechain and processed.