White balance settings are necessary because ‘white’ light isn’t plain white at all. Different light sources and lighting conditions have different colour temperatures: indoor lighting is warmer (more orange) than daylight, though daylight itself is warmer at dawn, turns cooler (more blue) in the course of the day, and becomes warmer again towards sunset.
Your brain compensates automatically, so that you perceive colours as pretty consistent; and your camera’s Auto White Balance setting is designed to do the same.
However, it can get it wrong, overcompensating when you don’t want it to and coming up with unusual hues in mixed lighting.
You might be lucky: your camera’s Auto (or preset, for that matter) WB setting may have been spot-on, so your image appears perfectly neutral when you open it in Adobe Camera Raw with the default As Shot setting applied.
However, considering that Auto settings aren’t infallible and camera presets are generic, you’re likely to have to make some minor tweaks at the very least.
Go to the White Balance menu in the Basic tab, and try the Auto setting first; if that doesn’t do the trick, try the preset you think best represents the lighting conditions under which the shot was taken.
The presets (not available if the shot isn’t a raw file) correspond to settings found on most cameras, and have the same effect on the image as choosing that setting in-camera would have done.
Flicking between As Shot, Auto and the presets will give you a fair idea of what works, and you can fine-tune with the Temperature (blue/yellow) and Tint (green/magenta) sliders.
Alternatively, you can click on a suitable tone with the White Balance Tool.