Only users who have Edit permission can input the values.
On the Issue Screen, Jira has two modes ( Issue Create, Issue View).
In Issue Create mode, Edit permission is required. The users can input the values in the custom field.
In Issue View mode, the users who have View/ Edit permission will see the value of the custom field.
Click on this custom field to go to Edit mode.
If the users don’t have such permission
Field Security Text ( Single - Line) gets the value from the inputted custom field on the Issue. The application will process the value (encode to store it, decode to show it).