Ohhhh. This is the "preload" for the stem/headset. What you do is loosen the 2 stem bolts (first pic) then tighten that top bolt just enough so the headset bearings don't have any play in them (hold the front brake and rock the bike back and forth, you should not feel any "clunk" or anything rocking). You want the bolt to be tight enough for no rocking, but not tight enough to feel resistance when steering. Then you tighten the 2 stem bolts to clamp everything together.