You've got a close, special bond with your car (or horse, or helicopter, or whatever). You call it by a pet name, spend hours polishing it with a diaper (or combing out his mane, if it's a horse).
This schtick comes in two versions - the unique ride, and as vehicle specialization. If you pick the unique weapon, the schtick applies to a single, actual ride, if your signature ride is a purple Cadillac with zebra print seat covers, fuzzy dice and a genuine mink steering wheel, you only get the bonus in that particular purple Caddy: No other Cadillac (no matter how purple) gives the bonus. Unique Signature Rides have a form of script immunity, a villain may steal if from you, but it will always pop up somewhere later in the story. It will never become permanently lost or destroyed unless the GM develops a plot around it. A unique signature ride is very precious, and can be held hostage if someone manages to steal it.
Simple Vehicle Specialization means you get the bonus with any ride of the specific model. It does not give the vehicle script immunity, but if you do lose your ride, you can replace it with an identical model.
A signature ride has three Fortune points stored in it, that you can only use when driving it. You don't have to use those points to actually drive the car; they can be good for other things you do while driving, such as impressing chicks. You just need to give a vaguely plausible explaination.