Mystcraft BoP Disabler is a very small mod meant to "fix" the current incompatibility between Mystcraft and Biomes o' Plenty for Minecraft 1.12. If you have both of these mods installed, Biomes o' Plenty biomes will spawn correctly in vanilla dimensions, but they will come out flat in Mystcraft dimensions. This mod disables the Biomes o' Plenty biomes in Mystcraft's symbols.cfg so that they do not spawn in Mystcraft dimensions.
In order for this mod to work, Minecraft must be completely started once with both Mystcraft and Biomes o' Plenty installed so that the symbols.cfg file can be populated by Mystcraft. If you are starting Minecraft for the first time with all three mods, you will need to restart for this mod to do its thing.
If, at some future time, the Mystcraft/BoP incompatibility is fixed, you can re-enable all of the BoP biomes through the Mod Options menu or the config file (mystcraftbopdisabler.cfg).