JPA needs to access the data from your object to persist them. It can access the data in two ways, FIELD and PROPERTY access. FIELD access accesses the class fields (attributes/instance variables) directly, normally through reflection or byte code weaving. PROPERTY access accesses the data through calling a get/set method on your classes, also normally through reflection or byte code weaving.
For field access you annotate your class fields. For property access you annoate you get methods.