Your post said "Combo Box" -- so I assumed you were using a combo box control (which allows the user to select or type in an entry).
It may help you understand the XPath filter to think of it this way: Your secondary data source of PopulateEmployees has a lot of rows. You need to get a value from the Column1 field for one of those rows. How can you tell InfoPath which row it is? By telling it the EmployeeID for the same row. Your Main Data Source has a field that has an EmployeeID in it. The XPath says "Please provide me with a Column1 value from this row, the row where EmployeeID is equal to the ID of the item selected by my user"