It allows you to "call a ride" when you are away from a shuttleport or starport.
This can be especially helpful when you are visiting Planets that have little to no shuttleport access (Endor, Yavin IV, etc). It can save you the long trek back through dangerous territory!
There is a minimum amount of "clear area" needed for the vehicle to "land", so you may have to move around a bit to find the right spot. Also, you cannot call it in most NPC cities or PVP zones. It cannot be used on Kashyyyk.
When you activate the ability, a 30 second timer starts to count down. (Combat will cancel the timer.)
Upon completion of the timer, an ITV (Vehicle) will appear.
Click on the Vehicle and choose the "Use" option.
This will open a map showing all of the places you can travel to on that planet (shuttleports and starports).
Remember that only the person that called the ITV can use it - no passengers!
Once you have selected the destination, click "Travel" and you will be given a confirmation window reminding you of the cost of the ride.
Each ride will cost 10,000 credits and 1 Heavy Fuel Cell - Mark V. (so make sure to keep some in your inventory!)
Accept that and away you go!
There is a 30 minute cooldown between uses, so plan accordingly!