How does the justice league fit into the dc extended universe?

Quick Answer
The Justice League is intended as a crossover film within the DC Extended Universe, a shared universe of DC superhero movies, but it has been delayed and never released as part of the current DCEU lineup.
Explanation
The DC Extended Universe (DCEU) is a media franchise of superhero films distributed by Warner Bros. Pictures, built on characters from DC Comics. The DCEU is structured around shared plot elements, settings, and characters, allowing crossovers between films. Warner Bros. began planning such crossovers as early as 2002, with director Wolfgang Petersen slated to helm a crossover between the Superman and Batman franchises. A Justice League film was initially planned to bring together multiple DC heroes, but it was put on hold in 2008. The commercial and critical failure of the 2011 film Green Lantern led Warner Bros. to scrap the original universe plans, further delaying the Justice League project. As a result, while the Justice League remains an envisioned part of the DCEU narrative, it has not yet been realized within the current set of DCEU films.