If the VSC warning light in a 2007 Prius comes on, what is the problem?

The VSC light in Your 2007 Toyota Prius stands for "Vehicle Stability Control", which is similar to traction control from other manufacturers. From time to time, You'll see the light come on during normal operation -- particularly, if You're going through an area where You, predictably, have poor traction. This includes icy roads, loose gravel, sand, or perhaps even exceedingly wet surfaces. The light's illumination can also indicate a fault, though, so that's something You'll want to be cognizant of. To try a reset, You can find the button with the same symbol as the light either by Your shifter or steering wheel. Put the vehicle in park, and hold the button for a few seconds. The VSC light should toggle. If it does not, then Your VSC system likely has a fault, in which case You should begin a diagnosis for repair.