AngularJS form elements look for the required attribute to perform validation functions. ng-required allows you to set the required attribute depending on a boolean test (for instance, only require field B – say, a student number – if the field A has a certain value – if you selected “student” as a choice)
As an example, <input required> and <input ng-required="true"> are essentially the same thing
If you are wondering why this is this way, (and not just make <input required="true"> or <input required="false">), it is due to the limitations of HTML – the required attribute has no associated value – its mere presence means (as per HTML standards) that the element is required – so angular needs a way to set/unset required value (required="false" would be invalid HTML)