For years, the Infinity Stones were the most mysterious of MacGuffins for fans of the MCU. Appearing as early as Captain America: The First Avenger (2011), the stones themselves have appeared in various forms throughout the history of the Marvel Studios franchise, seemingly changing as the story has needed. Things all came to a head in Avengers: Infinity War and Avengers: Endgame, when the stones were used to rid the universe of half of all life and then subsequently used to put them all back together. One dedicated Marvel fan has put together a handy infographic showing each of the stone's roles in the first scenario.
In the post that has since gone mega-viral on Reddit, u/Excelsior1001 suggests each of the stones had their own specific role in "The Blip," as in-universe characters call it. The Mind Stone was used to discern what was already living and not while the Reality Stone shed the physical bodies from this plane of existence. The Soul Stone was then used to "properly" disperse the souls remaining on earth while the Space Stone was used to execute Thanos's task across the universe at once. Lastly, the Time Stone aided in the quickness of the act while the Power Stone helped power all the other stones to make sure they could complete the task at hand.
At the beginning of Endgame, fans quickly found out Thanos has used the stones to atomize the items so that no one could retrieve them and undo what he'd done. That's what ended up sending the Avengers on a time journey to gather stones from times in the past so they could snap everyone back into existence.
As it stands now, the Infinity Stones still technically exist in the present timeline per se, it'd just take some magic or sorcery to gather the atoms of the stones and piece them back to physical objects.
Upcoming Marvel Studios projects include Black Widow on May 1, 2020, The Falcon and The Winter Soldier in Fall 2020, The Eternals on November 6, 2020, WandaVision in December 2020, Shang-Chi and the Legend of the Ten Rings on February 12, 2021, Loki in Spring 2021, Spider-Man 3 on July 16, 2021, What If…? in Summer 2021, Hawkeye in Fall 2021, and Thor: Love and Thunder on November 5, 2021, and Black Panther 2 on May 6, 2022.