An interpolation type, TCB is an acronym for Tension, Continuity, Bias, the three qualities which define the shape of the curve if you plot the value of the interpolated parameter against time.
Using the TCB interpolation type will fit a smooth curve between adjacent waypoints, much like the bline tool fits smooth curves between adjacent bline vertices.
There is some mathematical discussion of TCB here.
- Tension: How sharply does the curve bend?
- Continuity: How rapid is the change in speed and direction?
- Bias: What is the direction of the curve as it passes through the keypoint?