Grass jelly is made from Mesona chinensis, a type of plant. It's believed to have some effect on lowering the "fire" in your body.
Grass jelly is consumed differently depending on which country you're in. It can be enjoyed by itself, as a base eaten other with yummy things, or added as a side with other desserts. Depending on the season you'll find it in cold and hot form.
This is the most common form of grass jelly, in cubes and eaten with sugar water. I personally hate eating it alone
I'd rather have it with shaved ice and other type of desserts during summer.
Or in the winter, same concept except the grass jelly is heated. It then melts together and can also be paired with other ingredients shown in the shaved ice picture.