A gall is an unnatural plant growth triggered by a fungus, bacteria or insect infection. It is the plant equivalent of a cancer growth. In this case I think the gall has been formed by a scale insect, that lives inside the gall.
My fault. The question is not literal, without context. The 'what' refers to "This" in the first comment. As in...what is RogerF saying is a gall? (As I cannot see a gall in the photos - but then MM has said 'in this case'...perhaps the gall just needs to be pointed out to me)