By specifying how much alcohol you drunk last night you can estimated if your body has consumed all alcohol in your blood. That is, if you can drive or not.
The best is of course to leave the car the day after, but how long do you really need to wait before you can drive a car the day after? With the help of "Can I drive" you can in a easy way calculate how long it takes for the body to burn off all the alcohol.
Just remember that it is only an estimate and you would do best to leave the car even the day after.
Do not use this app as an absolute source for when you can drive a car the day after, if you have been drinking alcohol, its always best to leave the car the following day.