It’s just the value given by [ANALOGUE [x]] for the channel the sensor is connected to, on a scale within the range 0-255.
More description and code snippet for a 4tronix LDR Crumb here: http://4tronix.co.uk/blog/?p=921 (under the heading LUX/LIGHT)
That’s taking your question at face value – an LDR isn’t what I would first think of using for line-following, but that doesn’t mean you are wrong.
