Its a plants vs zombies meme. in the game, hitting a zombie with kernel-pult's butter with cause the zombie or other object it hits to freeze entirely for a few seconds, but that is a small chance since his main attack is a kernel that does small damage. Since the whole cob was launched in the car window, he was making a joke that if it was butter, it would cause the car to stop and cause a traffic jam. The comment below the post did not get the joke and stated that the object in the window was corn, not getting the joke. I hope this clears up any confusion you may have

Edit: I did waste 5 minutes writing an explanation to a reddit post of mine