Start with the pump lid O-ring.

Yes, air bubbles in the return jets usually mean a suction-side air leak. And nine times out of ten, that leak is a dry or dirty O-ring on the pump lid.

Turn off the pump. Remove the lid, wipe the O-ring and the groove clean, then apply a thin layer of silicone lubricant (the pool stuff, not WD-40). Re-seat the lid and tighten by hand—don’t wrench it. If that doesn’t fix it, check the skimmer weir door: make sure it’s not stuck in the open position, letting a vortex suck air. Also check the drain plug on the pump housing.

If you still see bubbles, you might have a crack in the pipe underground or a bad union. That’s a bigger job. But most people stop at the O-ring and fix it.