Disciplinary action is steps taken by the Employer to reprimand you for behaviour or performance issues.
Disciplinary action and the procedure that the Employer should take are covered by your Contract of Employment, a Company Handbook and/or the ACAS code of practice.
You can obtain a copy of the ACAS code of practice at www.acas.co.uk
The Employer should follow the ACAS procedure as a minimum in carrying out any disciplinary action, and a failure to do so could mean that they have behaved unreasonably and unfairly.
If your Employer indicates that they will be taking disciplinary action against you the first thing that you should do is request a copy of the disciplinary procedure and written reasons for the disciplinary.