This is a gall.  Galls are deformed growth on plants that occur in response to some insects and diseases.  Galls on branches are a serious problem since the deformed growth can eventually kill a branch.  Usually galls of this type are not treatable and if it is throughout the tree is not possible to trim them out.  It often shortens the life of the tree.  If it is not possible to remove the galls by pruning this tree may not survive.