It is the Torah in the form used for public worship.
A Sefer Torah is a handwritten Torah scroll
Britannica defines the Sefer Torah as the first five books of the Bible written in Hebrew by a qualified calligrapher on parchment and kept in the synagogue ark.
That form matters. The scroll is not printed, casual, or interchangeable with any bound volume.
It is used for public reading
Britannica notes that the Sefer Torah is used for public readings on Sabbaths, Mondays, Thursdays, and festivals.
This makes it central not only as a sacred object but as a living liturgical instrument.
The scroll is treated with visible reverence
Britannica describes the solemnity with which it is removed from and returned to the ark. That ritual care reflects how Jewish communities honor the Torah as more than text alone.
Why it still matters
The Sefer Torah still matters because Jewish public worship is built around hearing Torah from a sacred handwritten scroll within the community.
The shortest accurate answer
A Sefer Torah is the handwritten Hebrew Torah scroll used for public reading in synagogue and treated with exceptional reverence.