Panda assigns a so-called “quality score” to web pages; this score is then used as a ranking factor. Initially, Panda was a filter rather than part of Google's ranking algo, but in January 2016, it was officially incorporated into the core algorithm.
Google's Panda Update is a search filter introduced in February 2011 meant to stop sites with poor quality content from working their way into Google's top search results. Panda is updated from time-to-time. When this happens, sites previously hit may escape, if they've made the right changes.