A tape leader is the strip of plastic or paper spliced to the start and end of every reel. It records nothing. It threads the machine without touching the music, protects the first and last inch, and separates the songs with clean, visible silence. Right behind it came the reference tones, 1 kHz at 0 VU, to line up the machine that would play it back.
An assistant did all that before the mix engineer walked in, and wrote every channel on the masking tape along the desk. TapeLeader does that work today, on the session you just received, so you start with the music.

