What is $$phase in AngularJS?

Question

I found this code snippet which is part of a angular directive someone wrote for bootstrap modal.

//Update the visible value when the dialog is closed                                                                                                                                                                                                            
                //through UI actions (Ok, cancel, etc.)                                                                                                                                                                                                                         
                element.bind("hide.bs.modal", function () {                                                                                                                                                                                                                     
                    scope.modalVisible = false;                                                                                                                                                                                                                                 
                    if (!scope.$$phase && !scope.$root.$$phase)                                                                                                                                                                                                                 
                        scope.$apply();                                                                                                                                                                                                                                         
                });  

I understood that this part is for the latter half of two way binding we bind to hide.bs.modal event and update modal when UI changes.

I just wanted to know why is the person checking $$phase for scope and rootScope before calling apply ?

Can't we straightaway call apply ?

What is $$phase here?

I tried searching a lot, couldn't find any good explanation.

EDIT:

I found where I saw the example: Simple Angular Directive for Bootstrap Modal

Answer 1

$$phase is a flag set while angular is in a $digest cycle.

Sometimes (in rare cases), you want to check $$phase on the scope before doing an $apply. An error occurs if you try to $apply during a $digest:

Error: $apply already in progress