I believe that it is pretty self explanatory. What swastik said is true and is probably the most basic form of reverse sweeping but to add a more in depth explanation, it is when your opponent sets up, you cripple that setup Pokemon with either sleep or paralysis and then setup on your own
Here's an example. Take a Volcarona setting up against everyone's most hated durian, Ferrothorn. However, instead of switching out, Ferrothorn instead uses thunder wave on the quiver dance and now, you can use that chance to switch to a Pokemon that can kill take any of Volcarona's hits and kill it back
An even more concrete example would be Garchomp and Volcarona. Garchomp switches in on the quiver dance as it can take Volcarona's +1 attacks from high health. At that point, Garchomp can basically reverse the sweep by taking the incoming attack, hitting it with scale shot and outspeeding the next turn, or just outright killing Volcarona with stone edge
A special mention of reverse sweeping, and one that was prominent in gen seven, was how Mega Alakazam completely screws over weather sweepers. Mega Zam has trace, meaning that it copies its current opponent's ability, and if it switches in or mega evolves against something like Mega Swampert or Excadrill, you suddenly outspeed all of their mons and Mega Zam hits really hard, effectively reverse sweeping by turning your opponent's play style against them
That's how I understand it anyway
Here's a video one one player pulling off a reverse sweep. It's kinda outdated but it should give you the basic idea
https://www.youtube.com/watch?v=M53J0a2PwFw
