White smoke coming out of a vehicles exhaust is an indicator that your engines coolant is getting into your oil and then being burned through combustion as it makes it's way past your engines pistons.
Engine Coolant usually gets burned by your engine due to a bad head gasket or even a warped engine head.
If your car is showing white smoke coming out of the exhaust you need to get it to a mechanic to check it because your engine could overheat and even seize up and leave you stranded.
A cracked engine block as well as a damaged cylinder head can cause white smoke to come out of your exhaust.
Either way it's a serious engine issue and needs looked into and fixed before it causes major damage.