It is where argument becomes method.
The Gemara is the commentary on the Mishnah
Britannica defines the Gemara as rabbinic commentary on and interpretation of the Mishnah.
That definition is short, but it gets the core right. The Gemara is not a separate rival text. It is the analytical layer that turns the Mishnah into a living field of study.
Together with the Mishnah it forms the Talmud
Britannica explains that the Talmud is made from Mishnah and Gemara together.
That is why the Gemara matters so much. Without it, the Mishnah remains dense and compact. With it, the Jewish legal and interpretive tradition unfolds through debate.
The point is not only answers
The Gemara matters because it preserves the reasoning process itself. Questions, objections, comparisons, stories, and distinctions all become part of the text.
This is one reason Talmud study feels different from reading a code. The Gemara keeps the work of thought visible.
Why it still matters
The Gemara still matters because rabbinic Judaism does not only care about conclusions. It also cares about how conclusions are reached, tested, and argued.
The shortest accurate answer
The Gemara is the rabbinic commentary and discussion on the Mishnah that, together with it, forms the Talmud.