The @property keyword declares the property in the @interface section, and if necessary, adds an instance variable of the same name and type to the class behind the scenes. The @synthesize statement then causes the compiler generates both the getter and setter and (if not set to be a readonly property) the setter methods for that property.
You can still use the same code to access the value as before, and you can use dot notation as an alternative (and more natural) way to access properties of an Objective-C class:
MyClass* instance = [[MyClass alloc] init];
int val2 = instance.value + 10;
instance.value = val2;
// this still works and does the exact same thing:
int val = [instance value] + 10;