This is the hose that goes from the by-pass union under the intake to the #3 pipe stainless pipe on the rear of the timing cover. Well hidden, so it's often ignored. If you've never replaced it, it's probably time.
Re-use your spring clamps or order them with it if someone has already installed the worm-drive shenanigans. We don't recommend worm-drive clamps for these due to the small diameter. The spring clamps never need adjustment and will last a lifetime.