5.8Lookup relationships

Imagine:

Contact
   │
   │ AccountId
   ↓
Account

That's a relationship.

Salesforce commonly models relationships through:

Lookup

and:

Master-Detail

For your interview, the most important practical point is:

A relationship field stores/references another Salesforce record.

Example:

Contact.AccountId

references:

Account.Id

Likewise:

Opportunity.AccountId
Case.AccountId
Case.ContactId