P45 meaning tax document provided to an employee by the employer when they stop working. It contains the details related to salary and taxes paid till date in the tax year. At the start of a job, one may have to give the P45 to the employer which shows he/she is allowed to deduct the right amount of tax from the salary. Without this form, one may end up being on an emergency tax basis. By law, you should ask the employer to get the contract P45 explained where the employer sends details of part 1 to the HMRC and Part 2 and 3 is given to the new employer.