Grounding crystals remove negative energy from you. They also remove negative energy from around you and the crystal. This includes negative thoughts, feelings, emotions, intentions, vibrations, and more. They are called grounding crystals because all grounding crystals are connected strongly with the earth. Once they remove the negative energy from you or its surroundings, it then transfers the negative energy into the earth which neutralizes the negative energy entirely and makes it neutral or positive instead.
Black Tourmaline is an especially revered grounding crystal. It not only excels at removing negative energy, but it also makes negative energy neutral and puts it back out.