If you are an Adsense publisher, you will sometimes see PSA on your website.
Here is an article from PCHere Blog explaining everything you should know about PSA.
“ Google adsense has become a very popular tool for making money from your site by showing advertisements. But sometimes you see PSA’s – better known as Public Service Ads on your pages. These ads are displayed because adsense has not been able to find suitable ads for your page. ”
Source here
