The rings will be used "for the optical measurement of biometric data", especially your blood oxygen saturation. Aside from rings being much more compact and easier to transport than a watch, they're also said to take more accurate measurements than the watch.
The device would work by emitting light through the skin and blood vessels to the photodetector located on the other side of the ring. The light will essentially show the percentage of oxygen present in your blood.
If they do hit the market, these Fitbit rings would be direct competitors to the Oura ring, which can measure your respiration rate, heart rate, and temperature. According to the patent, the ring would send the data to the Fitbit app on your phone, and this would be done via Bluetooth.
Of course, the Oura ring is quite expensive, with a minimum RRP of $299, and we'd expect Fitbit to offer a much cheaper alternative to that.
Alternatively, if a smartwatch is more up your street, why not try the Fitbit Versa 2? It's widely considered one of the best Fitbit watches on offer.