As the name suggests, garages can repair, clean and change color and the color type of your vehicle. You can just drive into the checkpoint of garage and the services menu will open up.
You can refill the vehicle's boost charges (NOS) and vehicle repair kits are available as well should you need it for future use incase the vehicle is on the brink of breaking down in the middle of nowhere. Also If you want to purchase a trailer for your truck for deliveries, this is where you can obtain one.
You can find garages across the map with the following icon: 
