NHS means National Health Service.
The National Health Service in the United Kingdom includes NHS England, NHS Scotland, NHS Wales, and the affiliated Health and Social Care in Northern Ireland.
The NHS is important for you to obtain a job especially if you want to do a residency training for a specialty. As a fresh doctor in the UK (if you have completed your internship year), you will be appointed as an FY2 doctor (Foundation year 2) before you can go ahead with a specialty training.
For more info, visit the NHS job site below.