Tailgating has two different meanings, both of which come from the tailgate present on many vehicles such as light trucks and station wagons:

• The practice of driving closely behind other vehicles on a roadway. Sometimes used as a form of intimidation, this is considered to be an unsafe behavior, as the trailing driver has little or no time to react if the lead driver needs to make a panic stop or other maneuver.
• The (often celebratory) practice of eating and socializing. This may include barbecueing and is usually related to sports fans preparing for American football games while perched on the open tailgate of a vehicle (though many people participate even if their vehicles don't have tailgates).
• It's also a term for something which is not popular among corporate security offices, i.e when you follow someone else who has a keycard through a security door