A herbivore is an animal whose diet is made up entirely of vegetable matter. Many fish are herbivorous, including some closely related to the Piranha!
In the wild herbivorous fish would get their food by eating algae, plant leaves, or fruits and nuts and in the aquarium they should be given a diet which is made up largely from vegetable material.
Foods such as washed lettuce leaves, shelled peas, or cucumber can be fed to these fish and there are a number of algae and vegetable based dried foods also available.
Most fish that are considered herbivorous will usually benefit from a little meat in their diet so it is best to feed these fish with a normal diet that is supplemented with vegetable matter, rather than a strict vegetable only diet.