This Optifine feature changes how grass textures behave, here's a comparison between Better Grass ON and OFF:
The configuration file for Better Grass must be located at /assets/minecraft/optifine/bettergrass.properties.
Here's a template for said file:
# List of blocks that Better Grass will be active on:
grass= <true/false>
dirt_path= <true/false>
mycelium= <true/false>
podzol= <true/false>
crimson_nylium= <true/false>
warped_nylium= <true/false>
# List of blocks covered by snow that Better Grass will be active on:
grass.snow= <true/false>
mycelium.snow= <true/false>
podzol.snow= <true/false>
# Allows transparent grass texture to be used as overlay for the grass side.
grass.multilayer= <true/false>
# List of textures used by each block when Better Grass is enabled.
texture.grass= block/grass_block_top
texture.grass_side= block/grass_block_side
texture.dirt_path= block/dirt_path_top
texture.dirt_path_side= block/dirt_path_side
texture.mycelium= block/mycelium_top
texture.podzol= block/podzol_top
texture.crimson_nylium= block/crimson_nylium
texture.warped_nylium= block/warped_nylium
texture.snow= block/snow