Thanos, after eventually acquiring all six Infinity Stones for the Infinity Gauntlet, snaps his fingers and kills half of the life in the universe (including many Avengers). Before he disappears, Fury pages Captain Marvel.
Meanwhile, in one postcredits scene of Ant-Man and the Wasp, Hank Pym, his family, and Ant-Man try to help Ava Starr. Ant-Man collects enough particles in the Quantum Realm but gets stuck there when his friends disappear after Thanos's finger snap.