Research Slot
A research slot is the number of teams a nation may have actively researching at any given time. A nation will get one tech slot for free. It will gain one extra research slot for every 20 base Industrial Capacity they currently possess, up to a maximum number of 5 research slots. In other words, if your Industrial Capacity is >= 80, you will have the maximum number of slots available.
More information on research can be found on the Research page.