Apple tree canker?
It looks like your tree has burr knots. These are masses of root initials (new roots forming). They can occur in some types of grafted apple trees, especially in wet, high humidity conditions and if the tree was planted too deeply. We don't typically recommend any type of treatment for them, but you should be aware that they can cause structural problems or serve as an entry point for diseases and pests. There is more information about burr knots and what causes them, here on the following two Extension pages.