Recolours the picture by mapping brightness to a colour ramp. Dark tones take one end of the ramp, bright tones the other. It is the quickest way to make a greyscale generator look like something.
The palettes are not stored as lists of colours. Each one is four numbers per channel fed to a cosine wave, an idea of Inigo Quilez's, and the whole gradient falls out of that. It means a rich smooth ramp costs twelve numbers rather than a lookup table, and that every palette is smooth by construction.
Not all of them are computed in ordinary colour, either. Some run the same waves through a perceptual colour space instead, so their ramps move evenly to the eye rather than lurching between the corners of the colour cube.
repeat runs the ramp through several times over the tonal range for banding, and rotation cycles it.