DNA marker consists of a range of DNA fragments of known sizes (in number of base pairs)
In gel electrophoreis, you run your gel with the marker so you can calculate the the size of the fragments of your sample.
The relative mobility is the distance travelled by the DNA fragment divided by distance travelled by the bromophenol blue dye (or any other fragment if you can't see your dye front).
The relative mobility of any linear DNA is inversly proportionate to log10(size of DNA fragment)
You are supposed to measure the distances travelled by the fragements of DNA marker and tabulate the relative mobilities, and their log10(size)...then you should plot a straight line of log10(size) against relative mobility to obtain a straight line and the equation of the straight line
From the equation of the straight line, you can derive the length of DNA fragments from their relative mobilities...