Our mouths produce saliva to chew and swallow and maintain healthy gums and teeth, but the amount and consistency of saliva can vary considerably, from clear and free-flowing to thick, stringy, sticky or foamy. If you find you regularly have foamy saliva, it's probably a sign of dry mouth.
Definition of foam at the mouth 1 : to produce foam from the mouth because of illness or excitement The dog was foaming at the mouth. 2 informal : to be very angry about something He was foaming at the mouth with rage.