EA's “On the House” promotion is basically a 100%-off sale, with the intent of periodically bringing you back to their store. Whatever you acquire is free forever, but you only have a handful of days to claim it. Even if you're not interested in downloading it at the moment, it's good to poke in, press download, and just not actually download it until later. Maybe you'll buy something, too, while you're there. Either way.
This time is Battlefield 4: Naval Strike. If you have Battlefield 4, but do not have the Premium subscription, then this is your chance to grab a portion of its exclusive content for free. As the name suggests, it includes four, navy-focused maps, a hovercraft, and a new game mode. If you've played 2142, you might remember the Titan mode, where you would capture missile launchers throughout the map to weaken a flying carrier, and eventually destroy it. Similar idea, but with an aircraft carrier.
Also, the Westwood-developed action RPG, Nox, is “On the House” as well.