Specifications for LED strips - power supplies + installing in automobiles
LEDs need a minimum voltage to work, but once they get that minimum voltage they "turn on" and then the amount of current flowing though them controls how bright they are. Too much current and they blow out, so you need to limit that current to the amount that gets the desired brightness level (and is below the maximum). 

You'd think you could control that current by changing the voltage, but it is very hard because a very small change in voltage results in a huge change in current. Imagine trying to control the current flowing though a wire by changing the voltage across it - it would be very hard. 

So there are two common ways to control the current though the LEDs....


Use a current driver. This is a device that constantly adjusts the voltage to the exact level to generate a specified current. Keep in mind that this voltage is not fixed so it is not like a typical voltage power supply. 
Put a resistor in series with the LEDs. Because the voltage drop across a resistor changes with the current, this accomplishes sort of the same thing- as the current though the LEDs rises the voltage dropped across the resistor also goes up which lowers the voltage across the LEDs with lowers the current. These two effects balance out and settle on a limited current level. 


The resistor is much simpler, but less accurate (brightness changes with input voltage and temperature) and you also waste power in the resistor. For small amounts of current, people typically pick the resistor, but as you get to larger currents the resistors get big and hot, so a current driver usually makes more sense.