Linking a field in Agrivi means assigning that field to a particular crop, in a particular time period. This allows Agrivi to recognize which crop was grown on which field during a certain period of time.
When you link your fields, you select from which date this field is assigned to this crop and till when. This way, you can automatically monitor crop rotation per fields in case of annual crops.
If you are linking a field for a permanent crop, the date needs to be the date on which the crop was first planted (for example, 10 years ago) and the "Unlink date" section can remain empty.
To check how to assign field to a crop production, click HERE.