Hi Akash,
Reference selector is used to select the value of a one-to-one or one-to-many association by selecting the associated object.
So when you place a reference selector in dataview, the object retrieved by this data widget must be at the many end of a one-to-many association, or at either end of a one-to-one association.
For more refer this, https://docs.mendix.com/refguide/reference-selector