Like warp(), but applied several times over at increasingly fine scales, which gives the folded, marbled look of stirred paint or oil on water.
The difference from simply layering several warps is that each pass distorts a picture that has already been distorted. The coarse pass moves things a long way, then the next pass, working at half the size and half the strength, pushes the already-moved result around again, and so on. Detail therefore follows the shape the larger swirls imposed rather than sitting on top of it, and that compounding is what produces the characteristic folds.
octaves sets how many passes. Two or three is usually plenty; more quickly turns everything to mush. displacement sets the strength of the first pass, and each pass after it moves half as far, so the contribution stays roughly the same size on screen at every scale. freq sets how large the coarsest swirls are.