While most of the time you probably could use a LookupMaps to set up an entity references in you import maps, you may come to situation there you want put actual ID's (references) in your CSV file. So, the tricky part here is the format. To create EntityReference you need entity type name and it's GUID. How would you put that into CSV string? The answer is like this:
Note that surrounding quotes are required, because of the comma.