https://www.cnet.com/news/captain-marvel-postcredits-scenes-explained-avengers-endgame-infinity-stone-goose-pager-fury-tesseract/

The scene starts with a closeup of Fury's pager from the end of Avengers: Infinity War. It's still beeping and Captain Marvel's symbol is still on the screen. Suddenly the pager screen goes black. 

We then cut to Steve Rogers and Black Widow looking over some holographic images, still getting data on how many people are dead after Thanos' snap.