Relative luminometer units (RLU) have no unit. They are the result of the luciferase reaction and proportional to the expression of the luciferase gene, meaning the higher the numbers are, the higher the expression is.
To analyze these numbers, you usually use a control, to which your samples are then relative to. So expression from your luciferase vector is 10x higher in the treated sample compared to the control for example.
As the RLU has no unit, it is hard to compare measurements directly. To overcome this problem, you can use a second luciferase reaction (the renilla luciferase) to be able to normalize between samples and to correct for transfection efficiency, but still, your values are only relative.
They are influenced but the kit you use for the measurement, the machine you use (so always use the same machine) and so on.
To normalize the values, you divide each firefly luciferase value by the value for the renilla in the same well. Ideally alls renilla measurements are the same (as you transfected the same amount of plasmid in each well, have the same number of cells etc., but if not, this corrects exactly this problem). You cannot calculate the transfection efficiency from this control.
To calculate the relative expression ratios between your sample B and the respective control A, you set the control to 1 by dividing A/A=1 (for example: A=10, 10/10=1) The relative expression of B is then B/A= relative expression (for example: B=5, relative expression: B/A= 5/10=0.5). In the example the relative expression of sample B would be half the one of the control.
