Noise with a grain, like brushed metal or wood. Most noise looks the same whichever way you turn it. This has a direction, so you can point it and get streaks that all run the same way.
The trick is what it is built from. Instead of random values on a grid, the effect scatters thousands of tiny ripples across the picture and adds them all up. Each ripple is a little wave that fades out at the edges, and because a wave travels in a direction, so does the pattern. Give every ripple the same angle and the whole surface takes on a grain.
orientation sets that angle. isotropy decides how strictly the ripples obey it: at zero they all line up and the grain is strong, and as you raise it each ripple picks its own angle until the result is ordinary directionless noise again. bandwidth sets how wide each ripple is, so lower values give sharper, more defined strands, and density sets how many are packed in.