As the description states, Have 20 active drones then trigger a compression event. Be careful not to click any drones as it will destroy them prematurely and, at the time of writing this guide, the drone counter at "The Robotics" building displays the number of drones purchased during your current compression level not your number of active drones. This means if you have 20 active drones and destroy one by clicking on it, you will only have 19 drones but the counter in "The Robotics" Building.
It is recommended that you attempt this achievement before your first compression event as drone costs grow exponentially with each compression event. It is also recommended you have the "Bigger Pile" Perk active to prevent accidental triggering of the compression event.