These can be used in the mixer and the outputs screen. They basically define a relation between an input (along the x) and output (along the y).
Any point/node on the curve can be freely moved around to get the desired shape of curve. Up to 10 points/nodes can be defined.
The curve can also be optionally smoothed, although this feature should be used sparingly as it is relatively CPU intensive.
If more curve points are needed, a work around is to combine two curves by switching out one for the other in the mixer, with help of a logical switch.
Below are some examples demonstrating the use of custom curves.