The patent appears to cover a method of landing a rocket on a floating platform at sea. In their papers, SpaceX says that "by 2009, the earliest possibly priority date listed on the face of the patent, the basic concepts of 'rocket science' were well known and widely understood. The “rocket science” claimed in the ‘321 patent was, at best, 'old hat[.]'" Blue Origin has approximately three months to file a preliminary response to the challenge.
You can review the litigation documents here and here.
Disclosure: I run the website hosting several of the above documents."