Rootstock is a smart contract platform implemented as a sidechain of Bitcoin. It provides a similar functionality than Ethereum, but using a token that is pegged to Bitcoin - so it doesn't affect Bitcoin's value/price because it's not competing with it.

A sidechain is an independent blockchain, but it provides a "link" to the Bitcoin blockchain. In the case of Rootstock we are talking of a "pegged" sidechain. That means that the value of the token (unit of value, "coin") of the Rootstock blockchain is pegged 1:1 to Bitcoin's value (1 RSKBTC = 1 BTC).

How does this work? Well, first for every token created in the RSK blockchain a Bitcoin is "frozen" on the main Bitcoin blockchain. That means that they cannot be moved or used while they are frozen. So there are never more RSK tokens than frozen Bitcoins - the Bitcoins are "backing" the RSK tokens. This way, you can transfer Bitcoins easily to the RSK blockchain.

The trickier part is the other way around - "redeem" RSK tokens for Bitcoins. That is an important mechanism for the peg to work, because it ensures that you get a Bitcoin for a RSK token every time you need it.

For this to work, the RSK coins must be "burnt" or destroyed and the account who "froze" the Bitcoins on the main chain then can "liberate" the Bitcoins again. You destroy a RSK token and get a Bitcoin from this account.

This is not possible in a decentralized way still (there is a Bitcoin "opcode" or "command" missing), so RSK's strategy for their first release is to have a "consortium" of different organizations controlling that Bitcoin account that "freezes" and "unfreezes" the Bitcoins that are transfered to and from the RSK sidechain. These organizations then decide to "unfreeze" the Bitcoins if they detect the RSK tokens have been burnt.

(I hope that was simple enough and reasonably correct.)