A P60 is the form you get at the end of the tax year when you're working Pay as You Earn (PAYE).  It shows all the money you've been paid, along with the tax and National Insurance taken out of it.  It's a useful scrap of paperwork to have around, so keep track of it.  If you ever find yourself having to prove how much tax you've paid, your P60 is one of the easiest ways to do it.