The P11D form is issued by HMRC to UK based employers. It requires such organisations to detail the cash equivalent of benefits and expenses provided to any directors or staff during the tax year. This then informs HMRC as to how much Class 1A National Insurance your organisation needs to pay on all the expenses and benefits you've provided.
You'll need to submit this information, either electronically or via a hard copy, to your local HMRC office by 6 July after the end of the tax year. To complete your P11D, here are the items you'll need to consider (click on the image to view an enlargeable PDF version):