Mainnet swap as the name suggests refers to the transition of a cryptocurrency project from a pre-existing blockchain network such as Ethereum to its native one. When a crypto project is at its text phase, it uses the third-party blockchain network.
Once the project moves to its mainnet phase, the users generally receive the native tokens by swapping with the third-party ones. This is commonly known as a token swap or token migration.
The mainnet swap is basically this exchange of the third-party tokens to their native ones, at the end phase of a project. Sometimes the swap occurs automatically, while at other times, the user needs to manually swap them.