The soft, drifting, cloudlike texture most programs in this book start from.
It is made by scattering random values on a grid and filling in the space between them, so neighbouring places end up similar and you get rolling shapes rather than television static. Everything else is a variation on how that filling in is done.
type picks the method, and it changes the character completely. constant does no filling at all and leaves hard square cells. The others smooth it out to varying degrees, from a plain straight line between neighbours to curves that consult several grid points at once for a rounder result.
octaves lays the same pattern down again at twice the frequency and half the strength, which puts fine detail on top of large shapes. ridges folds the values at their midpoint so mid grey becomes the brightest value, turning soft hills into sharp creases. wrap makes the pattern tile, so its left edge carries on into its right, and speed sets it drifting.