Hidden grottos are areas where rare Pokemon (like stunky and clefairy) hide and they every so often give you items (bianca gives you a sort of tutorial on them if you pay attention) but there is a small chance that they will regenerate once you visit it. Other than that hidden grottos are in set locations (Like Post-team plasma giant chasm conflict there is one) and that's about all that they have to offer.
Hidden Grottoes are hidden paths between specific trees in the Unova region and can contain rare Pokemon or items to collect. If you find a Grotto then you simply go through the 2 trees in which the Grotto is located.
