The Mishnah is a collection of the Jewish oral traditions known as the Oral Torah. It was written down in the second century CE.
Under Roman rule, Torah study was restricted. Rabbis and others worried that the oral traditions would not survive. So Rabbi Yehuda HaNasi collected and edited these laws so that the learning would not disappear.
The Mishnah is not a code of Jewish law. It is like a study book of law. So most passages contain multiple opinions on the same issue.
The Mishnah consists of six “orders” or sedarim, each dealing with different aspects of Jewish practices. The Mishnah and the Gemara (a later set of commentaries on the Mishnah) together form the Talmud.
Text adapted from Simple English Wikipedia, Creative Commons Attribution-ShareAlike License 4.0.
Mishnah Torah, circa 1370 CE, Institut du Monde Arabe, Paris; CC BY 2.0.