Earn is the place where a portion of mStable's public
MTA is currently distributed. The other place is the "Vault" in Save. It is primarily built as a way to distribute Meta tokens and decentralise the governance community of the mStable protocol, while at the same time contributing to the utility of the protocol.
MTA rewards, users must contribute liquidity pool tokens from the relevant pool to the mStable Earn contracts.
Finally, it is important to state that participation in these liquidity pools can expose you to impermanent loss. In particular, pools containing volatile tokens as one or both sides of the pool are very susceptible to impermanent loss as the price of the volatile token changes. Please do your own due diligence on impermanent loss before choosing to use Earn. You can learn more about it here and here.