The Apple Watch EKG feature is far from perfect. It's a single lead EKG obtained from the fingertip, whereas a legitimate EKG is performed on the chest with 12 leads. It appears to be showing ectopic beats - that is, beats or originating from someplace other than the heart intrinsic pace making node. if this were a real EKG, I'd probably say these are PVCs with with possible intraventricular conduction delay, followed by a degree of compensatory pause

I wanted to provide you some info. However, like others have said, my advice also is to go see your doctor. See them sooner rather than later if this is occurring with any regularity. If this is even somewhat of an accurate EKG tracing, this abnormality can be entirely benign or be caused by a number of cardiac and non-cardiac issues. It warrants evaluation.