This option will only appear if a Performance review form is Visible to employees
Having the option to lock any review responses gives Manager, HR and Admin users the ability to decide when employees should see their comments within a review form.
In essence, a form can be completed by the employee and submitted to the manager, the manager can then lock the form hiding their response from the employee until they are ready for the employee to view them.
When setting creating a new form or editing an existing one within the form builder, you will need to make sure that Visible to employees is set to 'Yes', at which point the "Option to lock Form responses" setting will appear, you should ensure that this is set to 'Yes'.
Once an employee has completed the form, you will need to go to Performance > Review forms > View data against the form which you're looking to use.
You will notice a Padlock icon beneath the Actions column
Open padlock - This means that the form is active or open for the employee to review the manager's comments
Closed Padlock - This means that the form is inactive or locked meaning that employees cannot see manager's comments.