It's a punctuation mark, which means it's a symbol used in writing to clarify meaning and structure.
Commas have various roles in grammar, such as:
* Separating items in a list: "I bought apples, oranges, and bananas."
* Setting off introductory phrases: "After the rain, the sky cleared."
* Separating independent clauses: "The sun was shining, but it was cold."
* Indicating pauses: "He said, 'I'm leaving.'"
While commas contribute to the grammatical structure of sentences, they are not words or parts of speech like nouns, verbs, adjectives, etc.