If you want 5V out of a normal regulator you need to sent 7 volts or more into the regulator.
With a low drop out regulator you need only 6v or 5.5v into the regulator.
Drop out voltage = (input voltage - output voltage) where the regulator stops working well because the input voltage is too low.
I always wished they had let me write that app note back at national Semi explaining what dropout actually is...... so many really well versed people believe the above definition, but it is not true among semi makers.
For LDO regulators, dropout is defined as the input-output voltage differential when the output has dropped 100 mV below the nominal voltage. So, it's not the point where the regulator no longer works "well", it is WAY PAST the point where the regulator works AT ALL.
I would wager 99% of the people in the electronics industry don't know this... and I ended up talking to most of them. When a regulator is in dropout, it has no loop gain, no ripple rejection, no nothing..... it just looks like a resistor from input to output.
But an even more interesting fact: the regulator stops working "well" at input-output voltage differences a lot higher than dropout. For example: a "typical" LDO whose "dropout spec" is 0.5V might have 70 dB of loop gain at 100 Hz with an input-output voltage differential of 2.5V. At 1V differential (twice dropout spec) the gain may have dropped to as low as 30 dB. So, it's still a voltage regulator, it's just a poor one.
I once tried to publish some performance curves showing a family of curves for both ripple rejection and loop gain at different input-output voltages approaching dropout..... and was threatened with my life. Semi makers prefer to perpetuate the myth that regulators "work" right up until dropout... they don't actually. They gradually collapse.