A disciplinary policy is the rules set by your employer setting out their expectations of their employees around things like conduct; performance; and critically also then setting out the process by which those matters will be managed if an employee’s behaviour or performance fall short of those ‘rules’.
You can usually find your employer’s disciplinary policy in their ‘office manual’ which is generally incorporated into an employees terms and conditions in their contract of employment. If your employer changes or updates the disciplinary policy during your period of employment they should notify you (and all colleagues) of any such changes.