What does Harper mean by "class"?
The term class here has absolutely nothing to do with OOP classes. Rather, a class determines which possibility in a C union an object takes.

For example, one definition of a binary tree is: a binary tree is either a leaf or a pair of binary trees. Leaves form one (boring) class, pairs of binary trees form another.

Mathematical terminology often uses words which have other meanings in common usage. You should get used to that.