When you reset something, you start it over again or adjust it. If you accidentally set your alarm to go off at 3:00 am, you'll have to sleepily reset it for 7:00 am.
You may like to reset your car's odometer before starting a long trip, so you can keep track of your gas mileage. Or, you might tend to reset your alarm clock on the weekends, so it wakes you later. You can also reset many electronics by essentially making them go back to zero, or to a starting point: "I need to reset my cellphone to the original factory settings."