Sheetla Mata is the Swaroop of Maa Parvati.
Skanda Purana describes a legend related to Sheetla Mata, in which it is told that Sheetla Devi was born from Brahmaji. Sheetla Mata is considered to be the form of Lord Shiva's consort Shakti. According to the legend, Maa Sheetla Devi had come to live in the kingdom of King Virat on earth with Jwarasura(a demon made from the sweat of Lord Shiva).
Maa Sheetla became angry with this behavior of the king, due to the fire of Sheetla Mata's anger, red spots appeared on the skin of the king's people present in the Kingdom.

People's skin started burning with heat. Then King Virat apologized for his mistake, and after that, the king offered raw milk and cold lassi to Goddess Sheetla, then Mother Sheetla's anger calmed down. Since then the tradition of offering cold dishes to the Mother Goddess has been going on, especially on the Ashtami Tithi.
According to the Devi Mahapurana, when Jwarausra gave bacterial fever to the small children, Maa Sheetla arrived to purify the blood of small children and also freed them of the
fever-causing bacteria. Maa Sheetla Devi resides with Ashta Bhuji swaroop i.e Devi with Eight hands holding Trishul,
Broom, Chakra, a Jar of Water, Neem leaves, Shankh, and Vard Mudra.
Maa Sheetla Devi symbolizes devi ensuring overall Protection, Good health, fortune, and power.