A field of cells, like foam, cracked mud, or the scales on a lizard. It is usually called cellular or Worley noise.
The idea is simple: scatter points across the picture, then colour every pixel by how far it is from the nearest one. Pixels sitting right on a point come out dark, pixels stranded between points come out bright, and the boundaries land exactly halfway between neighbouring points. That halfway line is what gives you the cell walls, and nothing ever has to work out where the walls are; they appear on their own.
shape is the parameter that changes the character most, because it changes what "distance" means. The default measures it as the crow flies and gives rounded cells, but it can also measure to a diamond, triangle, square, hexagon, or octagon, which makes the cells take those shapes instead. cellSmooth rounds the joins between neighbouring cells so they melt into one another rather than meeting at a crease, and speed lets the points drift so the whole surface crawls.