Entry reference is an add-on which allows you to show sobipro entries in the details page of an entry .
It simply adds a field type called entry reference which allows choosing entries from any section in the add form .
Then the entries you have chosen in the add form will be shown in the details page.
It is very useful when it comes to linking between different sobipro section example a companies directory and products directory all you have to do is :
1- In companies section create a new field called : our products
2- In the fields configuration choose the products section as the entires source and thats it, On adding/editing a company you will be able to choose products for the company and it will be shown in the details view under the field : our products
please refer to the demo and the documentation for better understanding