Lets say your a freshman (car) in highschool. When the bell rings you've got 5 minutes to get to your class on the other side of the building. But since the bell rings for everyone the hallways are immediately packed with other students (air) and your little scrawny self has to push through everyone else (air resistance). You make it to class but your exhausted having to push through everyone to make it.

Now the next day you realize that one of the lineman (the truck) on the football team gets out of class right next-door and he's heading in the same direction. As he walks, the other students (air) move or get pushed out of his way and then eventually fill back in after a couple feet. You realize you can let him do the hard work of clearing out the other students and then your just follow him close enough that the other kids havent had a chance to fill back in behind him. Congrats! Your using his slip stream! You get to class using less energy!

Now as to why you dont want to do this. Lets say one day he stops to tie his shoe, you dont react fast enough and run into the back of him. He's big enough that he may not even realize you did, but for you? You just ran into a human wall, broke your nose, dropped all your books and split your drink.

Now picture that doing 75 MPH running into a semi. Most of the times you do it, nothing will go wrong and youll save a couple cents of gas. But the one time it does your going to spending 10s of thousands to fix the mistake. The risk verse reward is just not worth it.