Astronomers measure signals in terms of Janskys (1 Jy = 1×10-26 Watts/m2/Hertz, and the GBT often measures signals of milli- or even micro-Janskys. That’s a decimal point with 32 zeros after it:
0.00000000000000000000000000000001 W/m2/Hz
This is much less than, say, the energy of a single snowflake landing on the ground. Another possible way to look at this is that if you were to take your cell phone to Mars, its energy as seen here on Earth would be much, much greater than the energies frequently measured by the GBT. This is why it is incredibly important for the science we do that we keep human-made radio signals at a minimum.
Human-made radio signals are what astronomers call Radio Frequency Interference, or RFI. What is RFI? Watch this video (linked) to learn more!
The Green Bank Interference Protection Group is responsible for minimizing all radio frequency interference to scientific observations using telescopes at the Green Bank Observatory. This is done through regulating the use of equipment on site, developing hardware and software to protect our scientific observations from RFI, and through administering the National and West Virginia Radio Quiet Zones which protect all scientific instruments at Green Bank Observatory.
You can do your part by understanding the rules and regulations for visiting the site and for visiting the Radio Quiet Zones.