This mod adds friend codes to Minecraft, which are an easy way to play in the same world with remote friends. They can be created by selecting "Open To...", which replaces "Open To LAN", and selecting the "Friend Code" submenu. They can be connected to through "Multiplayer" -> "Direct Connect", which also gets a "Friend Code" submenu.
The mod is not strictly required on either end, as you can also set up the connections through the i2p router directly. It just makes things more convenient, supporting persistent codes tied to the world, as well as ephemeral codes tied to the play session. (World codes will not persist across different computers, however.)
This mod was created for a game jam with the theme not about you. It is not about you, the player; it is about showing alternatives to, in this case, Minecraft Realms, and more generally centralized systems, which tend to milk the userbase for money, while providing a subpar experience. After all, you can't run mods on Realms, and there's no guarantee they won't ban you for something you did between you and your friends, even when none of your friends want you banned for it, based on their interpretation of how they think you should manage your relationships. These alternatives may require active community participation and sharing of resources, such as by running I2P routers, but their benefits may be absolutely worth it.
Note: You will need an externally installed I2P router configured with SAM bridge. (Note for modpack makers: this is a soft-requirement. The game will still launch if the I2P router is not available, and trying to host/connect will display a relevant error.)