The Maker platform is how Dai stays valued at USD$1.
MKR is really a governance token for the overall Maker platform, acting as a second line of defense, it is responsible for maintaining the DAI stable coin’s steady value of USD$1. In fact, in December 2019 MakerDAO transferred control of its native currency, MKR, from the Maker Foundation to Maker Governance. This allows token holders to have more voting control on changes to its Protocol with a one-token, one-vote system. The two tokens’ values are algorithmically interlinked, with MKR tokens governing the cryptocurrencies if the amount of ETH locked up is insufficient.
This is a complicated process, but it works in a similar way to a conventional bank, who must only keep in reserve a small proportion of the money they have actually lent out.