The idea itself is pretty simple and straightforward. When you put the watch on, the sensors (placed on the forward edge of the watch or on the side of it) will scan your veins to ensure that it matches that of an image stored in its memory. Based on that, it will be able to identify that you are the owner and then unlock itself, or maybe open up features that might need authenticating.
The patent also suggests that the sensor could be used to detect the user’s pulse rate, which is apparently different from person to person. This sounds similar to what Apple is using right now. The Apple Watch authenticates the wearer using the heart rate sensor and skin contact on the underside of the watch, meaning that as long as they are wearing it, they will be able to make use of features like Apple Pay.
However if the watch were to come off, they would need to re-authenticate themselves again if they want to keep using the feature. That being said, whether or not this feature makes its way onto future Samsung wearables remains to be seen, but it is a pretty good idea.