Grass is a non-solid plant block that drops wheat seeds when broken. Its color changes depending on what type of biome it is in. Sometimes tall grass can spawn as a two-block high variant of grass. Ferns and tall ferns are variants only found in specific biomes but have the same characteristics as grass.
Grass on a grass block (via screenshot)
Grass and tall grass are generated in random patches on top of grass blocks in most Overworld biomes. It can be found commonly in biomes such as plains, forests, savannas, and lush caves. Fern and tall fern are generated only in jungle, taiga, snowy taiga, and old growth taiga biomes.
Grass and fern can also be found in loot chests in villages. You can also grow grass and fern by using bone meal on grass blocks, though it is not guaranteed that they will grow as sometimes flowers can also grow.
Grass naturally generated in plains (via screenshot)
Grass is mostly used as a source of wheat seed for farming. You can also place them around your home as decoration. You can also place them in flower pots. Grass, tall grass, and fern can be placed in composters where they have a 30%, 50%, and 65% chance respectively of increasing the level of the compost by one.
Wheat seeds dropped after breaking grass (via screenshot)
Grass is a great and reliable source of wheat seeds as it can be found quite easily. You can also use grass for decorating and to increase the level of the compost in composters.
A. Not only can grass be used to make a lawn appear as if it's thriving, but it can serve as a path border.
A. For grass to spread, it must have a light level of 9 or brighter directly above it.