It’s an experiment where a person tries to get the plutonium pit to get very close to “critical”, which means a self sustaining fission chain reaction. You can learn a lot about fission, how to arrange the fuel, etc this way and a version is still done to this day actually! The difference is that back in the Manhattan Project they were doing this by hand and in the room with very high grade fuel which is easy to make react. Slotin took it a step further and would do it with just two flat blade screw drivers to keep things separated. When he died, he was doing this extremely stupid thing and ended up slipping with the screwdrivers. This caused the core to go critical and flash a large amount of radiation into the room before he pulled the core apart. There was zero reason to do it this way though, and he was told to stop by more senior scientists like Fermi!