The footage, which appears in a collage of other footage from HBO shows, is very slim. The only new footage in the collage shows Jon Snow (Kit Harington) receiving a hug from Sansa (Sophie Turner). This is, apparently, from the upcoming eighth season of the show, but we don't know for sure. It might even have been filmed specifically for the purpose of a teaser. With GoT, there are no rules. There are only Reddit theories and potential HBO hacks! This appears among footage from HBO's upcoming 2019 shows Camping, Flight of the Conchords, Room 104, Pod Save America, Barry, Veep, and True Detective.
What does it mean? It means that, erm, Jon Snow and Sansa Stark are reunited (which happened during season 7) and they are both very, very worried. See the image below and then watch the full HBO 2019 teaser.