This battery level indicator is a small version of the ZTS-MBT1, which has more size and features than this scaled-down unit. This particular one can do "9V" (really more like 7.2V) NiMH batteries, but there is also another, much more popular version of the Mini that handles 3V lithiums (e.g. CR123A) instead.

As you can see, it consists of a bunch of terminals sticking out of the unit and a tethered test probe which you touch to a cell's negative terminal. It rests in a groove around the edge of the unit when not in use. If you look closely, there is a slight depression near the upper-right corner, to the right of the sine wave. That's where the On button is. Click it, and the green indicator LED (slightly above and to the left of the depression, to the right of the icon of the full cell) lights up. The unit will automatically turn itself off after thirty seconds or so, but if you want, you can open up the back and temporarily open the circuit.

As you can see, that's 4AAA. It's really too bad that you need to use cells to test cells with this model, but oh well.

It's pretty easy to remove the test probe from its nook on the top left side of the unit.

This and other real testers are immensely useful, revealing those $3 voltage testers as the sick jokes they are. These are more expensive, of course, but if you want to actually test the capacity of a cell, this is the only way to go. I'm not just using that as an expression here, by the way; those cheap testers don't tell you anything. They're such a waste of time.