When the required checkbox option is enabled the field will have to have a value before the record can be fully saved in the Data Viewer on the web app or the mobile apps. This is ideal if you wish to ensure that the data collector enters a value for the field.
On the web app, this means that the field will need a value before you can save the record.
On the mobile apps (Android, iOS), if records can be saved as a draft (allowable by default), then the user will be presented with an option to save the record in a draft state if any required fields do not have values.
When importing data, the importer does not enforce requirement logic and records will be created/edited without the value in the fields. The next time the record is edited after it has been imported the fields that are required will need to have values.
The Required field? option is available on all field types, except for Section, Label, Hyperlink, and Calculation fields.