What "primary" means
A contact can have multiple phone numbers and multiple email addresses. One of each can be marked primary. The primary phone number is dialed first when you start a call session on that contact. The primary email and phone number are displayed in bold on the contact profile to make them easy to spot.
How to set a primary phone number
Open the contact's profile.
Under Phone Numbers, find the number you want to make primary.
Click the three-dot menu next to that number.
Click Make Primary.
The number is immediately set as primary. The previous primary phone number, if any, loses its primary status. You cannot set a removed phone number as primary.
How to set a primary email address
Open the contact's profile.
Under Emails, find the address you want to make primary.
Click the three-dot menu next to that address.
Click Make Primary.
The address is immediately set as primary. You cannot set a removed email address as primary.
Notes
The Make Primary option does not appear for a number or address that is already primary, or for one that has been removed.
Setting a new primary phone number affects which number the dialer dials first for that contact. The dialer picks the primary number before falling back to others.