Standard Field Names connect the contact fields Act-On needs (name, email, etc.) with the field names used in your lists & segments.
The most important field in Act-On is Email Address. Depending on your list setup, this field could be named "email", "e-mail", "emailaddress1", or any number of variations. Standard Field Names tell Act-On where it should expect to find your contacts' email addresses by connecting the field name that you typically use.
To set up standard field names, see this page.