What does it mean to mark a contact as primary?
By marking someone as the primary contact, you are ensuring that they will receive all automated communications when the customer is assigned to a strategy.
It also means that all the tags related to the name (First & Last name) will pull from the primary contact detail for automated communication.
How do I know if the contact is a primary one?
The primary contact will have a blue star displayed next to their contact details.

Can I have more than one primary contact?
Yes, you can have as many primary contacts as you want, just remember that they will all receive the automated communication.