"Grass, grasshopper, snake, hawk" is a food chain* because there is a feeding hierarchy in which food energy flows from one energy to the next. Allow me to elaborate, the grasshopper feeds on the grass and consumes its energy, then the grasshopper is consumed by the snake, who is higher on the trophic** levels, and finally the snake is consumed by the hawk. The hawk absorbs all of the food energy collected from the previous organisms, completing this example of a food chain.
*A food chain is a chain of organisms that feed on each other in a systematic hierarchy and the food energy they collect is passed up the chain.
**A trophic level is a group of organisms that are on the same level in a food chain. So as one organism is consumed by another, it is being eaten by an organism on a higher trophic level than itself. This can be seen in your example, from the grass all the way up to the hawk.
