Around the world, the movement of trains is governed by signals placed next to or above tracks. One of the methods of displaying those signals is with colored lights, lighted in a particular order. Signals allows you to look up the meaning of a given signal aspect right from your iPhone or iPod touch.
Currently, Signals only supports colored light signals, using aspects and indications as specified in "NORAC Operating Rules, 9th Edition," by the Northeast Operating Rules Advisory Committee, which covers most of Amtrak's system, and several smaller commuter lines within the United States. In the future, we may expand to cover more rule books.
Note: this application is for entertainment purposes only, and should not be relied on for factual accuracy when driving an actual train. Hopefully, though, a certified locomotive engineer wouldn't need to look up signal indications while driving.