A yellow diamond with two curving arrows indicating opposite directions means:

An S-shaped or double-curve arrow on a yellow diamond warns of a winding road — a series of curves ahead, not a single curve. Reduce speed and prepare to steer through multiple bends in sequence. Single-curve signs (one arrow bending one way) warn of one curve, often with an advisory speed posted underneath. Winding road warnings are common in mountainous areas, along rivers, and on older state highways where the road follows the terrain rather than running in straight surveyed lines. Following distance becomes more important on winding roads because the vehicle ahead may slow unexpectedly at any bend, and you have less time to react if your view ahead is short.