A P-45 is a tax document issued in the United Kingdom when an employee leaves a job. It provides important information about the employee's earnings and the taxes that have been deducted during their employment. The P-45 is divided into three parts: one for the employee, one for the new employer, and one for the tax office. When starting a new job, the employee should give their P-45 to their new employer. This helps the new employer calculate the correct amount of tax to deduct from the employee's salary. The P-45 is essential for ensuring that the employee pays the right amount of tax and avoids overpayment.