PR (mostly UK) means public relations. In the US it would probably be publicity or working as a publicist. As Molly said, sometimes this involves creating a campaign to improve a person or company's public image, but it's also their job to generally communicate with press / the media about that person or company's activities. For example, their new products, offers, a new movie or a musical artist's new album.
Examples:
He does PR/publicity for Google.
I've been working in PR/publicity for 10 years.
A PR/publicist just contacted us to offer review tickets for upcoming Broadway shows.
(in the UK, 'a PR' can also refer to the person themselves)