Your tree's leaves have galls.  These are common on shade trees and have a variety of causes, usually insects.  They are almost all totally harmless.  Since your tree is healthy, these are certain to be the benign kind.  

If we knew the species of tree, we could be more specific about the kind of gall, but it's not really necessary to know.

Here is more about galls (there are many shapes--some pretty crazy!): https://extension.umd.edu/resource/shade-tree-galls

Ellen

Thank you. Can you tell what kind of tree it is from the leaf photo? I am having trouble identifying it in my tree book.

We would need to see additional traits to make a more concrete identification; aspects like leaf arrangement on the stem, shape of the twig scars from older growth, and more help in identifying trees with similar foliage. From what we can see in the photos, this resembles a Hickory (Carya); eight species are documented in MD, which can be viewed in this gallery: https://www.marylandbiodiversity.com/viewChecklist.php?genus=Carya

Miri