This is considered the initial stage of gum disease. When you neglect to brush or floss your teeth, bacteria builds up on your teeth and gums, creating a sticky coating called plaque. Plaque that is left for too long will harden into tartar, which cannot be removed by simply brushing or flossing.
From this stage, you will notice bleeding, swollen, reddened gums when you brush. If left untreated, gingivitis will progress into gum disease.