The Mishnah is a big, old collection of Jewish teaching that became written Jewish law. For a long time that teaching was spoken and remembered, teacher to student. Later, scholars wrote it down. That written collection is the Mishnah. It sets out how to live — food, rest days, family, courts, holy days — in short, tight lines that later books explain. The Talmud, which students of Judaism study at length, is built on it: the Mishnah is the base text, and the Talmud wraps discussion around it. You will meet the word in history, religion, and Jewish studies, not in casual chat. It is a proper name, so we capitalise it. Though actually, the Hebrew idea behind the name is “teaching by repeating”; the book is named for that way of learning.