“Torah” usually gets translated as “law”. It means something closer to “teaching”.
Five books, the first part of the Hebrew Bible: Genesis, Exodus, Leviticus, Numbers and Deuteronomy. From creation to Moses’ farewell, with Egypt and forty years in the desert in between. Christians have the same five at the start of their Old Testament (The Bible), where they’re called the Pentateuch; for Islam it’s the Tawrat, revealed to Moses (The Quran).
Tradition says Moses wrote it. Modern scholars see it as a text assembled from several earlier sources, which took its final form around the 5th century BC.
In the synagogue it’s read from a scroll written by hand on parchment by a trained scribe, where a single mistake can make the whole scroll unusable. It’s read in weekly portions, the full five books in one year, and when they finish, on Simchat Torah, they start again from Genesis. Jewish tradition counts 613 commandments in it; next to it there’s the Oral Torah, later written down in the Mishnah and the Talmud.