Most healthy adults will have blood oxygen level above 95%. Blood oxygen levels below a certain threshold are leading indicators of respiratory problems, and can impair heart and brain function. Similar to the Apple Watch’s existing low heart rate notifications or electrocardiogram features, the Apple Watch Series 6 could tell the wearer if it detects abnormal blood oxygen and suggest seeking medical help… More here.
This is far from a new feature, but in my experience fitness trackers and smartwatches are not good at monitoring this. I own a monitor (they can be bought for around £20) and it works perfectly so I’m not convinced they have huge merit in a watch. Then again, expect Apple to do something with it that builds it neatly into everything else in the Apple Watch.