Even though cats are obligate carnivores and don't usually volunteer to eat fruit or vegetables, they do like to eat grass.
Eating grass is a normal behaviour in cats yet not fully understood – the general understanding is that it helps to move food or hairballs through the digestive tract (either up or down as grass eating often results in vomiting).
It may also provide them with essential trace elements in their diet, so it is recommended that cats without access to grass outside are provided with a source indoors. This can be a commercial pack of 'cat grass' or a pot in which grass seeds or grass from the garden can be grown.
Indoor cats without access to grass may chew other potted plants they would usually ignore or avoid and which may be poisonous. Make sure you don't have any lilies in pots or vases in your home.
More information on Poisonous plants available here.