It matters because it took centuries of oral legal tradition and gave them durable written form.
The Mishnah is the first major rabbinic code
Britannica defines the Mishnah as the oldest authoritative postbiblical collection and codification of Jewish oral laws. It was compiled over time and given final form in the early 3rd century by Judah ha-Nasi.
That is why the text matters so much. It stands at the point where oral tradition becomes a stable, studyable legal corpus.
It does not replace the Torah
Britannica notes that the Mishnah supplements the written law found in the Pentateuch. It records interpretations and legal traditions rather than replacing Scripture itself.
This distinction matters because Judaism does not treat the Mishnah as a second Bible. It is a rabbinic framework for living the biblical tradition.
Later rabbinic study grew around it
Britannica explains that intensive study of the Mishnah by later scholars produced the Gemara, and together Mishnah plus Gemara form the Talmud.
That makes the Mishnah foundational not only for law but for the whole later culture of Jewish learning.
Why it still matters
The Mishnah still matters because it is one of the clearest places to see rabbinic Judaism organizing itself: classifying law, preserving dispute, and giving the Oral Law a structure that later generations could teach and debate.
The shortest accurate answer
The Mishnah is the earliest major codification of the Jewish Oral Law and the foundational rabbinic text on which later Talmudic discussion is built.