Goddess Kali’s tongue is a symbol of power. There are two different interesting stories about Kali’s tongue.
Rakta-bija and Goddess Kali
In Devi Mahatmya, Kali unfurls her tongue in her role as the ultimate deliverer called upon to salvage a situation that seems hopelessly out of control. She is summoned by Durga herself to destroy the demon Rakta-bija, whose name means ‘blood-seed’. The demon Rakta-bija had the magical ability to produce a double of himself instantly every time a drop of his blood fell to the ground. (Cell regeneration, Read this – 5 Biotechnology breakthroughs that were already invented by Hindu gods)
Having wounded Rakta-bija with a variety of weapons, Durga and her assistants, a fierce band of warriors known as the Matrikas, find they have worsened the situation: as Rakta-bija bleeds more profusely from his wounds, the battlefield gets filled with Rakta-bija duplicates.
Desperate, Durga summons Kali, who spreads her tongue across the battlefield, and swallows in one gulp, the swarm of blood-born demons and sucks the blood from the original Rakta-bija until he falls lifeless. Kali’s tongue here is a weapon, to be feared, a reminder that nature ultimately consumes all life.
In a world undergoing anarchy and evil, where men had become wicked and several demons (asuras and rakshasas) were reigning supreme, the Gods felt worthless; they were not being able to protect the world.
Goddess Kali and Lord Shiva
In a desperation, Kali (which means Black or dark) was summoned. Kali is supposed to be another avatar of Hindu Goddess Durga. Kali went on a killing spree and a full-on a rampage. She was ruthless and fearless in killing the demons. However, even after all the demons were killed, Kali’s anger didn’t subside. She was the most tangible form of destruction.
The Gods became concerned as to how to stop Kali. Then they went to Lord Shiva and asked him to do something about it.
Lord Shiva hatched a plan. He took the form of a big log of wood and was lying down on the road. So far everything Kali was setting foot on was getting destroyed. But the wooden log would remain the same, and this made Kali re-check things. At that very moment, Shiva came back to his God form.
Kali realized that she had stepped on Lord Shiva, which was a big mistake according to Hindu thought. You just never step onto anybody; if you do, then you apologize because God is supposed to be there in every human being, and putting your foot on them means you are being rude/disrespectful towards God.
When Kali realized her mistake, she put out her tongue in embarrassment (this is a common body language in India too), and stopped right there.
This is how Kali was stopped from her killing spree.