Think about this:
An S9 signal (50 microvolts) at the input terminal of an H.F. receiver with a 50 ohm input impedance, is 50 PICOWATTS. That's 50 TRILLIONTHS of a watt, for the prefix illiterate.
That's a STRONG signal.
An S1 signal, (assuming a standard of 6 dB per S unit) is down 54dB below that (almost six decimal places). That's in the QUINTILLIONTHS of watts!
(Thanks to KL7AJ on QRZ.com’s public forums)