The PPD is a skin test to test for tuberculosis. A 2 step process is required, initially, then a one step yearly. You get the PPD skin test (1st step), then 2 weeks later, you get the 2nd one (2nd step). Sometimes, if for instance you change jobs, and cannot provide them with proof of prior PPds, then they will have you take a 2 step.
this is a federal healthcare guideline.... it helps to protect the people that you (any healthcare worker) come in contact with. As fo why they require a 2 step, I believe that it is incase you have come in contact with a case of TB in recent days.