Tape Bias is a high-frequency signal (generally 100Khz) added to an input signal when it is recorded by a modern analog Tape recorder. Tape Bias increases the signal quality of most audio recordings significantly vs. "raw" recordings.
Early tape recorders simply applied the unadulterated input signal to the record head, resulting in mediocre recordings with poor low-frequency response. This is largely due to the non-linear response of magnetic tape at low field strengths. In 1940, J. von Braunmuhl and Dr. W. Weber discovered (accidentally) that the addition of a high-frequency tone kept the recorded signal in the linear-response zone, resulting in a striking quality improvement.