When people say "mainnet launch" in the Reserve community, they are referring to the launch of the full Reserve protocol on the Ethereum mainnet.
The team is aiming to launch the full protocol in Q4 2021. It's difficult to be sure we will hit this timeline in advance, because there are multiple levels of security that have to be achieved, and sometimes security analysis takes a surprising amount of time.
On a high level, the rules for the protocol and its decentralized governance must be well-designed, so that it can't be attacked economically or politically, and so that it will maintain the right economic properties over time.
On a low level, the code must be carefully evaluated to make sure it actually works as intended. Even audited contracts get hacked, so just doing an audit to check the box isn't sufficient to be confident in code security.
With that perspective on security in mind, the protocol team is working hard to execute the launch as soon as possible.
For info on the RSR release schedule at the point of mainnet launch, see here.