red02focus

i just recently bought a ford focus 2002 se, checked it everythings fine inspection passed said the car looks to have been taken very well from the last owner, i was driving and stopped at a light, was there for about a min, and when the light turned green i pressed on the gas and the car wouldnt go! i notcied that the car had turned off? the radio was still on so idk what could have happend, a week later again, i was at a light and i had a feeling it was gonna shut off on me again because of what happend last time, i figured it did that because it was standing still for a long time, n years ago when i was learning how to drive i remember one of my friends telling me not to keep their car (idk what his car was i dont remember) running while holding the brake that long cuz it will turn off. is there a reason for this?!