Without knowing any information whatsoever about your car. Thinking maybe its fuel injection. This button might be placed on the passenger side fender and is an inertia switch that shuts the fuel supply off while in the middle of your front end collision so fuel doesn't fly all over and make things worse.
1977 Blue Fiat Spider, Weber 44IDF's, Header, MSD
1996 Burgandy Eclipse GS-T Spider (turbo, convertible)
2007 Orange Eclipse GT-V6, Modified Slightly
2003 Econoline Conversion Van on 20's
1987 Alfa Romeo Milano Platinum
1996 Subaru SVX LSI AWD