English: “And he will give you a sign” — From a prophet you may demand a sign, but you may not demand a sign regarding the Torah, as it is written “according to the Torah that they will instruct you.”
On the daf / in context: A prophet who seeks to introduce something new may be tested by a sign; the standing Torah instruction of the court requires no such test.
Type of Learning
Derash on the verse (Sifrei) — distinguishing the need for a sign from a prophet versus from the fixed Torah.
What Is Learned
One may seek a sign from a prophet, but one may not seek a sign concerning the established Torah itself.