Accounts are businesses, 1040 accounts, etc; they are your client's businesses, tax returns, organizations, etc. One account can be associated with multiple contacts.
Jane Doe’s Flower Business is an Account. The Relationships tab shows you what Contacts associate with the Account. Jane Doe’s Flower Business may have the following contacts that are associated with the account: Jane Doe, Jim Doe, and John Doe. However, Jane Doe is the Primary Contact whereas Jim Doe is the Billing Contact.