generally means that is the maximum that you aren't supposed to go over. It is possible to go over that current, but the makers of the LED have deemed that it would do harm to the LED.
Directly hooking up the LED to a car battery would be bad without some sort of current regulation or at least voltage regulation.
If you send in a certain amount of current through an LED, the current will pass through the LED and the LED will consume some voltage. That voltage is called the Forward Voltage (Vf), and it varies, depending on how much current you send through the LED.
If you apply a certain voltage to the LED, it will draw a certain amount of current. Current and voltage affect each other.
Now, particular to LEDs, as the current rises, the Vf will rise as well. However, for a large current increase, the Vf only rises a tiny bit. This is why it is better to use current to regulate power to an LED than to use voltage to regulate power to the LED.
And if you are a graphical learner, check out the first graph on page 7 of this datasheet for a commonly used LED:
http://www.cree.com/products/pdf/xlamp7090xr-e.pdf
You'll see the relationship between voltage and current is exponentially proportional, and that the better variable to try to control would be the current.