In a Master-Detail relation, one feature class can have multiple relations to another feature class. For example, a master feature, such as a building can have several detail features. Similarly, each feature can have several labels.
Data Model: Master-Detail relation
In feature class forms, you can either show the detail features in a separate tab or you can display a reference button to open the related form and all related features in the filter. You configure this in Form Designer.
Data Model: Display of related features in feature class forms