The key is in the no cloning theorem. If Evil Eve intercepts some of the signal that Alice is sending Bob she will resend incorrect information 50% of the time.

In a classical bit stream, ie loads of photons in a fiber or a radio wave, Eve can intercept 10% of the signal without Alice or Bob knowing, and she can recreate and send that 10% perfectly.

Alice and Bob can publicly announce measurement bases and a few of the bits to see if the line was touched.