Bubbling just means it's bubbling. It isn't necessarily related to fermentation or gravity changes--changes in temperature, air pressure (weather fronts moving through), physical agitation, and a whole host of other things can make an airlock bubble.
Use the hydrometer to gauge fermentation. Use the airlock to keep air out.
On deck: Little Bo Pils, Bretta Off Dead (Brett pale)
Secondary: Oude Bruin, Red Sky at Morning (Sour brown ale)
On tap: Saison Duphunk (sour), Amarillo Slim (IPA), Earl White (ginger/bergamot wit)
Bottled: Number 8 (Belgian Strong Dark Ale), Eternale (Barleywine), Ancho Villa (Ancho/pasilla/chocolate/cinnamon RIS), Oak smoked porter (1/2 maple bourbon oaked, 1/2 apple brandy oaked)