Post-credit scenes are nearly as much a tradition in superhero movies as capes and exaggerated melodrama are. Popularized by Marvel in all of the MCU movies, DC as jumped on the train, with post-credit scenes in movies such as Suicide Squad, Wonder Woman and now Justice League. This movie actually has not one but two post-credit scenes, which you can watch below (before they get taken down from YouTube).
Spoilers: The first scene is light-hearted and seeks to answer a question nearly all comic book fans have debated with their friends at one time or another: who would win in a footrace between The Flash and Superman? Apparently Superman is the only being not affected by Flash’s super speedforce, as he is the only one not frozen in time by the effect. Superman appears to be just as fast as The Flash. Unfortunately, we don’t get a concrete answer, as the scene ends just as the footrace begins.
Scene #2 checks in on Lex Luthor, who has apparently broken out of prison and is living the good life on a yacht. Pulling up next to that yacht is none other than Deathstroke, who gets a proposition from Luthor to join together and create “a league of their own.” Unfortunately, as with the first scene, we don’t see the actual answer.