A reference field is any field in Salesforce that "'refers to" another object that contains its own fields - whether that be an Account, Lead, Contact, etc. They are most easily identified by containing a link that allows you travel to that Object in Salesforce if clicked.

In this case, the Account is referenced on the Opportunity:

Displaying fields from other Objects comes in handy for a variety of reasons. Let's say you want to show Account details when an Opportunity stage changes, or the Company/Phone Number/Email of a Lead on an alert that is triggered when a new meeting is set. Being able to tap into fields on referenced objects allows you to do this.

It's easy. When setting up a workflow on app.troops.ai, the new field selector lets you add in fields from references Objects. The referenced objects are displayed on the left-hand sidebar.

Here, I've set up an Opportunity alert to fire whenever a Stage moves to Intro, and I've included some information from the Account. 

Users in Salesforce on objects (e.g., an Opportunity owner) are also referenced objects - you have not only the person's name but email, phone number, etc. Just make sure to select what you want to show.

You get Inception-ed and can never escape. Just kidding - in that case, if that field is selected to show on the alert, whatever is being displayed on the field in Salesforce will be displayed on the Troops alert as well.