The fettle/ curves function provides various YUV, RGB and HSL graphs from which individual channel curves (i.e red, green, blue, yellow, cyan, hue and satuation) can be modified.
Each graph indicates the curve of input colour (the horizontal axis) in relation to the output colour (the vertical axis).
Control brightness, contrast, hue, etc. by independently adjusting the interaction of YUV,RGB or HSL using the 27 grids provided. By adding multiple control nodes and dragging the spline curve to affect multiple colours within a single secondary.
The video below provides a practical example of the uses of fettle in grading: