The syntax ?<name of class> creates a meta.class object, which is an object that contains information (metadata) about a MATLAB class. In this context, as the value for a property's Access attribute, they indicate that only methods of certain classes (the classes whose meta.class objects you specify) are allowed to get and/or set those properties.
This technique of restricting property access is a bit of an advanced maneuver, but the examples on this page may be simple enough to show you what's going on.
