The Black One; a form of Ma Durga

Kali is a black-skinned goddess who emerged from the body of Ma Durga to defeat the demons. She wears a garland of skulls (or severed heads) around her neck and a skirt of severed arms–both symbolizing the sense of egotism. In one hand she wields the sword of spiritual wisdom (prajna) and in the other carries a severed head (ego). Despite her fearsome appearance, her two other hands are held in the gestures (mudras) that indicate: protection and blessing

Apart from the terrible nature, another interpretation of Ma Kali stretching her tongue, skull, and finger belt is as follows

The tongue represents the speech. Ma Kali is the Devata of speech.

Skulls represent intelligence and memory. Ma Kali Bestow in her devotee intelligence and memory

Fingers represent talent and writing skills, Ma Kali helps her devotees to win the world with talent and presenting skills.

This is the reason why Kalidasa went to Kali temple for Vidya.