Fields on your forms can be created and customised as required, or left as optional.
A required field means the form user won't be able to complete the form until there's data in that field. Required fields will be indicated on the form with a red asterisk. If this field is left empty, it'll be highlighted in red when attempting to submit the form.
An optional field is on the form for users to provide information already known, or if they want to provide additional information. They won't be forced to enter data into this field, and the form can be completed with this field left empty. Optional fields are indicated on the form with 'optional' written alongside the field label.
To mark a field as required, click on the relevant field in 'editor', and select the checkbox alongside 'Required'.
You'll notice that some fields are automatically set as required and can't be changed. These are core system fields, such as 'First Name' and 'Last Name'.