To make the field read only and display a data value that is calculated and displayed through an attached script, select Calculated – Read Only. Users cannot edit the calculated value.
To make the field editable and display a data value that is calculated and displayed through an attached script, select Calculated – User Can Override. Users can edit the value if the calculation script has been written to accept the input. If a user does edit the calculated value, the custom message you specify in the Override Message box appears.
See also 

To define the behavior of the field