The Primary Domain field in site settings accepts multiple urls. The first domain entered into this field will be the only one that displays to the world.
Any subsequent domains in that list will redirect their results to the Primary Domain.
Before you can populate the field with your domain, the domain needs to be configured. Read more on this here.
If the field is left empty, the Primary Domain will default to a secondary domain which is established when the site is set up - usually something like e.g abc.engagementhq.com. This secondary domain on most occasions is superseded by a more marketable domain such as yoursayabc.com.