Certified GKH Gardening Expert
Those are galls! They are caused by fungal and bacterial infections, specifically spread by insects. They won't cause serious issue to the tree, but you may want to remove as many as possible in order to allow the trees to be as productive as they can. You will also want to treat with insecticides to prevent the issue.
These articles will help: https://www.gardeningknowhow.com/special/organic/what-are-organic-pesticides.htm