Hey Sara,
The inferred fields are based off of IP addresses. It is the systems best guess as to where the person is, but keep in mind it is not perfect. These fields show up under the Marketo information section (Sales Insight, MSI) on the Lead and Contact layout section.
They are a part of MSI, so they are required. The other fields that you definitely want to make sure are synched are First Name, Last Name, Email Address, and Company Name. Obviously, you will need more than that, but those are the most important.