Explanation: When exposure metering is active (if not, tap the shutter release button) the label ISO above the button matters and you can alter the ISO sensitivity of the camera sensor.
Possible values are from ISO 100 to ISO 25600 but ISO 12800 is displayed as „H 1.0“ (H for „high“) and ISO 25600 as H 2.0. But despite this naming they are nothing else than the next higher ISO sensitivities in the row.
Higher ISO sensitivity means shorter exposure time (=less risk of unsharp pictures because of subject movements and camera shaking) but also higher noise (=worse image quality).
The actually used ISO sensitivity can be higher or lower than the value you have chosen manually if the „Auto ISO sensitivity control“ is switched on in the shooting menu ISO sensitivity settings.
Tips: My standard setting is ISO 200 and the automated ISO sensitivity control that you can switch on not via this button but only in the shooting menu ISO sensitivity settings chooses a higher sensitivity if needed.
That fits for most situations when shooting hand held and I intervene only from time to time e.g. when I want a shorter exposure time for moving subjects.
The picture quality of the D7000 at higher ISO values is better than its predecessors and really disturbing only at the very high sensitivities ISO 12800 (H 1.0) and above. But please try yourself, noise is visible already before and you have to balance your need for short exposure time and your expectations for optimum quality (=low noise).