Lerner explains that RSK is a smart contract platform that adds a Turing Complete Virtual Machine — a “world computer” — to Bitcoin. Additionally, RSK achieves 100-300 transactions per second on chain and confirms most payments in less than 20 seconds, while leveraging Bitcoin's proof of work security.
RSK is a Bitcoin Sidechain — a separate blockchain that carries a separate token but is able to exchange tokens back and forth with the main Bitcoin blockchain. Users can move bitcoin to the RSK chain, make use of the converted bitcoin, and then eventually move their tokens back, as bitcoin, to the main blockchain.
To put bitcoin back into the main Bitcoin blockchain, a RSK transaction fee has to be paid. There is no concept of mining for coin creation, since all coins come from the Bitcoin blockchain, but there is the all-important concept of merged mining: miners can run both blockchains and receive fees for validating bitcoin transactions as well as for running RSK contracts. It is expected that merged mining should provide sufficient incentives to miners.