What is atrial fibrillation? Is it serious?

Depends. Atrial fibrillation is a very common arrhythmia, often unclear in origin, but usually due to enlarged upper heart chambers (atria). High blood pressure, heart valve problems, heart failure, hyperthyroidism, coronary disease, alcohol, or even spontaneous in normal people. Some drop in energy may be noted, but one can live normally with it. Blood thinners to minimize strokes important if persists.
AF. Af is a disorder of heart rhythm in which the electrical activity of the upper chambers becomes chaotic, contraction stops, & the ventricles beat too rapidly. In your age group, most people feel better with restoration of normal rhythm. Medication can control the rate and correct the rhythm; if not, electricity will. If necessary, ablation can correct it. Blood thinners are sometimes needed.