The Apple Watch Series 6 has a blood oxygen monitor for measuring oxygen saturation levels (SpO2). As we said in our review of the smartwatch, the feature is not meant to replace your doctor. The measurement is only a "key indicator of your overall wellness" according to Apple.
Another way to take a reading: put your finger directly under the blood oxygen sensor, making sure to cover it entirely and hold steady. As tech YouTuber Dave2D, who also works with medical equipment as his day job, notes in his video, taking an SpO2 reading this way is often as accurate as a pulse oximeter. He said he didn't have as many failed readings as on his wrist.
Again, it's unclear if there's any real practicality to the Series 6's blood oxygen monitor. For most people, an SpO2 reading isn't valuable since it doesn't suggest any direct correlation to any specific health issues. But if you just got a Series 6 and can't figure out how to get an SpO2 measurement, now you know.