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The oxygen sensor measures the amount of oxygen in the exhaust system to determine if the engine needs more or less fuel. A regular oxygen sensor is sometimes referred to a narrow band o2 sensor (NBO2) because it is only accurate at stoich (14.7air fuel ratio (AFR)) which is where an engine will produce the least emissions. As you can see by the image below, a NBO2 is only good at telling you if your are rich or lean but never how rich or how lean you are.
Since 14.7 AFR where it is accurate is such a narrow range and almost impossible to hold it there the acutaly AFR bounces back and forth from rich and lean, if you graph the voltage output it looks quite a bit like what siesmograph or lie detector needles sketch. When the ECM is using the O2 sensor to correct the fuel tables it measures how long it is lean and how long it is rich, if they are equal then the AFR is at 14.7 right where it should be. O2 "counts" is how many jumps back and forth it makes.
Due to its nature a NBO2 is all but useless for PE (power enrichment, hard acceleration) where the most power is made with a richer then stoich. You can tell that you are rich but not how rich which leads to more difficult tuning, which is where the WBO2 comes into play...
Since 14.7 AFR where it is accurate is such a narrow range and almost impossible to hold it there the acutaly AFR bounces back and forth from rich and lean, if you graph the voltage output it looks quite a bit like what siesmograph or lie detector needles sketch. When the ECM is using the O2 sensor to correct the fuel tables it measures how long it is lean and how long it is rich, if they are equal then the AFR is at 14.7 right where it should be. O2 "counts" is how many jumps back and forth it makes.
Due to its nature a NBO2 is all but useless for PE (power enrichment, hard acceleration) where the most power is made with a richer then stoich. You can tell that you are rich but not how rich which leads to more difficult tuning, which is where the WBO2 comes into play...