Croutons are often added to a salad as a topping to give some extra texture and flavor. Croutons are often made by re-baking bread or sauteing bread until its crunchy and then it is cut into cubes and seasoned.
Croutons are very popular on caesar salads and also they are sometimes added to soups or stews.
Additionally some people eat them straight as a snack.
So can guinea pigs eat croutons at all?
Yes, but only if they are plan and have no additives. As well you should only let them nibble on a few. Giving them too much can make them sick.
It is best to make home made croutons though so you know there is no preservatives or additives included in the food.
Also it is important to note that croutons have no nutritional value for guinea pigs and therefore should really be feed this food on purpose.
Maybe as a quick treat but not as a main source of their diet. There are far better foods for guinea pigs to eat.