When people say "mainnet launch" in the Reserve community, they are referring to the launch of the full Reserve protocol on the Ethereum mainnet.
You can follow along with our progress towards mainnet launch on the mainnet dashboard, which shows the work items that have yet to be completed, as well as a progress bar towards "code freeze". The team is aiming to launch the full protocol as soon as humanly possible without sacrificing on our level of security.
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.
For info on the RSR release schedule at the point of mainnet launch, see here.
As we are building in public, you can follow the progress of our protocol engineering team on GitHub, see here for our latest commits.