The put call ratio refers to the number of put options divided by the number of call options. For example, the total trading volume of SPY call options on a day is 300k, and the total trading volume of SPY put options is 200k, so the put call ratio is 200k/300k=0.67.
This indicator can be seen as a reverse indicator. If most people choose one direction, then reverse thinkers will take the opposite direction. Taking the above example as an example, most traders are bullish, then the reverse thinker will treat it as a bearish signal.
Under normal circumstances, the number of call options purchased is always greater than that of put options.