Stray light bouncing around inside a camera lens: the bright core, a horizontal streak, an optional star, a rainbow ring, and a chain of coloured blobs marching across the frame.
Everything is added on top of the picture rather than blended into it, because a flare is light arriving at the sensor that should not be there, and light only ever adds.
The chain of blobs is the part that makes it convincing. In a real lens these are reflections bouncing between glass elements, and they always fall on the straight line running from the light source through the middle of the frame and out the other side, spaced by the geometry of the lens. That is what this does too: each blob sits at a fixed point along that line, with different spacings for different lens types. Move the light and the whole chain swings around the centre as one, which is what reads as optics rather than decoration.