The oil light or indicator can be one of two things depending on year and model of your vehicle. It can be an indicator of low / no oil pressure, or low oil level.
If this light comes on, the first thing to do is shut off the engine and check the oil level. If the level is ok, then you need to have an oil pressure test done by a repair shop. Then they can diagnose the reason the engine has no / low oil pressure.
Low / no oil pressure will usually result in engine noise, and eventually engine failure.
This may be the most important light or indicator on the dash.
More Auto Repair Answers, Questions And Help
Electrical Testing Belt Diagrams Mustang Engine Overhaul All about Tires