Mandatory fields are fields that are required to have a value in order to save a record. They determine the minimum amount of information that is required to save a record. If a mandatory field is not filled out on a record, the user will not be able to save that record, and will be prompted to fill out the field.
We recommend that you limit the number of mandatory fields. Marking too many fields as Mandatory can make using the application for yourself or your employees more tedious than helpful.