It is signal that you need more water in your daily intake. Also there are some meds that cause this dry mouth syndrome.
I've been drinking about 1 cup of water per day or less for the last 35 years. I've always thought I get enough water through my pasta sauce and moisture in my bread. Only this last week I've increased my consumption to 1.5 L of water per day as a challenge. I use the washroom almost twice per hour and even have to wake up 4 or 5 times during the night to pee.
I say all this to preface that I've never had a dry mouth until this week when I'm drinking a lot of water.