A flash flood occurs when there's a great deal of rain in a very short time. In the Phoenix area, we periodically experience flash floods during monsoon thunderstorm activity.
The Phoenix area has improved the urban drainage systems over the past two decades, but this is still a desert with a sprawling set of cities in it, and heavy rains don't drain quickly. Streets flood, and otherwise dry rivers and arroyos can become death traps in a matter of minutes.
Flash flood warnings will be issued on both local radio stations and TV stations. It's best not to be outside when these heavy rains are expected, since visibility is poor. When was the last time you checked your windshield wipers? Many people discover at exactly the wrong time that the heat has dried and cracked their wipers, rendering them useless.
Finally, don't drive through big puddles or through washes when there's water present. Sounds silly to say, but every year people are stranded in their vehicles just because they were sure they could make it (and were wrong).