The lights and gauges in the dash are the way the vehicle communicates with the driver. There are many different lights indicating different things such as the battery light, ABS, check engine, etc. One light in particular is more critical then the others, that would be the oil light.
If you're saying to yourself "why is my oil light on". It's indicating that the motor has no oil pressure. This could be caused by one of two things. Either the oil pump has failed or the motor simply does not have enough oil. In either case, vital components within the motor are being starved of their much needed lubrication. The longer this continues the more critical it becomes for the motor. When it carries on for too long the motor will seize and shut off.
Once this happens something in the motor has failed, usually meaning the motor is in need of repairs if not replacement. For this reason the oil light in the dash is one of the most important lights to pay attention to. Once illuminated the quickest way to solve the problem is to check the dipstick with the engine off. If it's low simply fill with oil until it reaches the full line or dot on the dipstick. If the motor is already full of oil and the light is still on then the fix is more complex. At this point contact your mechanic immediately before any severe damage is done. A single repair is much cheaper then a full motor replacement.
Not all oil lights are the same. Some lights tell you to change your oil which is not the critical light we're talking about. If you're uncertain about what yours looks like refer to your owners manuals for an accurate understanding, but generally the light looks like the picture here.