The field guide highlights the field on the invoice (using a blue box) that you are currently verifying. The field guide is divided into three areas:
On top is the field on the invoice itself.
In the center is the interpreted or entered field value, in plain text. This allows you to easily compare the value on the invoice with the value interpreted by INVOICES (or entered by you).
On the bottom is the name of the field.
Value
Date
Name
You can turn off the field guide by deselecting View > Field guide. If you do this, only the value of the field on the invoice itself is highlighted.