Ok er, its telling me the voltage in the battery? which in turn is (on the same scale) telling me if the battery is good or needs replacing. For £2 this better be highly accurate!
Most of the traditional battery testers drain current from a battery and measure the voltage under load. For Alkaline batteries, this is a indication of remaining battery life. However, its going to be different for ni-cad, and nimh batteries, so unless the battery tester has a battery type selector switch, or specifically tests a particularly type of battery, then assume its only accurate for alkaline or conventional batteries, and then, only for a particular cell type.
I have one that provides 4 different loads so I can use high loads for "D" cells and light loads for AAA cells. The green, yellow, and red bars are only accurate for conventional batteries, but it will read volts as well.
Its only useful to tell battery condition in very broad terms, and I'd never use it on a rechargeable battery.