As the Nine Realms begin to converge in the film's final act, a number of portals are all visible at once. The presence of a world bathed in fire seem to make sense - surrounded by one of lush greenery, one of ice, and one of rock - but comic fans know that what's being glimpsed is far more important than just a balance of elements.
The fiery realm seen for just a few seconds is in fact Muspelheim, the home of Surtur and his army of Fire Demons. Standing over 1,000 feet tall, and capable of unleashing unstoppable attacks with his Sword of Doom, Surtur is as close to a walking apocalypse as you can get. With power even Thor can't challenge, this tiny window into his world posed more of a risk than most will realize.
Several Marvel executives and crew members have implied that Surtur and Muspelheim will play a role in the future, so if that's the case, this simple nod foreshadows Thor's greatest enemy.