Field-Level Security (FLS)

The concept of Field-Level Security (FLS) is easily pictured in the FLS settings for a given profile's access to an object:

You can define Read Access and Edit Access on fields (Edit Access requires Read Access). If you remove Read Access from a given field, the user won't be able to see that field on the object layout, even if the field has been added on that layout.

The same applies to Edit Access. If the record is in edit mode but the current user doesn't have Edit Access to that field, the field won't be writable.

Required fields (marked as required or master-detail fields) will always have read and edit access, while system fields (such as Created By or Created Date) will always be read-only.

If you want to see what determines field access, jump to Setup | Object Manager, look for any object, click on Fields & Relations, select any field, and click on the View Field Accessibility button to see the following display:

For every record type (if any) and profile, you will have a picture of field accessibility (Hidden, Read-Only, or Editable) on the assigned profile.