Other forms: hitting the road; hits the road
When the car is all loaded up, everyone has made a last visit to the bathroom, and all the doors are locked, your mom might say that it's time to hit the road: She means you should get in the car. You’re leaving on your trip.
Hit the road usually does literally mean "start a trip by road," regardless of whether it's going to be just a half-hour drive home or a month-long journey across the continent. But it can also simply mean "leave," especially when used to tell someone to go away, as in "Hit the road, dude, and stop bugging us." Then it's rude slang, like saying "Get lost!" or "Get out of here!"