An "open-plan office" is where employees aren't given offices or even cubicles, but instead work on desks in a big, open room. 

Someone on Twitter pointed out that...

"Open-plan offices have been found to reduce productivity and impair memory. They’re associated with high staff turnover. They make people sick, hostile, unmotivated, and insecure. Open-plan workers are more likely to argue more with their colleagues."

I think of a spectrum of productivity scenarios ranging from the "open-plan office" on one end of the spectrum, and being in the middle of an Italian vineyard on the other. That's MY idea of an "open-plan office"