Visit https://coros.us/mods/desirepaths for a better showcase of the mod
As you walk around, grass blocks are slowly worn down to eventually reveal where all living entities walk most, like players, monsters and animals, villagers, etc. Will slowly regrow over time if not continually walked on.
I recommend turning on "Force threaded chunk rendering" in "Client Settings" in Minecraft Forge Mod Options Config. My mod keeps block updates to a minimum but this setting still helps a lot.