My galaxy watch detected signs of afib. after that reading, every other reading was normal again. how worried should i be that this is something serious?

If you do not have any symptom at that time, just disregard that one time reading. If you have symptoms such as palpitation or shortness of breath then you are more vigilant for the next episode. If symptom(s) and abnormal rhythm recur then you should see your doctor then or see a cardiologist directly if allowed. If you are healthy otherwise, that transient rhythm reading is likely an error.

Putting health monitoring apps into every smartwatch around demonstrates the ability of these devices to pick up false-positives. If you are otherwise well and feeling well, monitor your own health and bring it up to your physician at your next visit. If you are have repeat reading, and symptoms like lightheadedness, palpitations, pain, get evaluated sooner than later.