Excessive drooling, called 'hypersalivation', can indicate a dental problem like an abscessed tooth, infected gums, sores in the mouth, a scratch on the roof of the mouth, something stuck between teeth or between teeth and gums, or even tonsillitis. You can read more about hypersalivation on this page:
It can also indicate nausea, perhaps from acid refulx, and drooling is also a symptom in toxic ingestion.
If your dog is acting normally in all other respects, re: eating, drinking, eliminating, playing, active/alert, etc., that's great, but I'd recommend a vet visit, ASAP, to identify the cause of the drooling, after a thorough, hands-on examination. Once the problem has been diagnosed, the most appropriate treatment will be prescribed.
If your dog should be found to have acid reflux, ½-1 tablet daily, of Pepcid AC (regular strength) should help, but please don't give any medication without first consulting your vet.
I hope your dog will be fine!