If it is like most DSLRs, depending on which release mode you have set, you may still need to press the shutter button for each exposure. What Auto Bracketing does is automatically shifts the exposure for several successive frames. Most cameras will bracket for three exposures. The default pattern for most cameras is 0, -, + but you can customize it in some models to go -, 0, + or +, 0, -. So once you have it set up you take three pictures in succession and the camera brackets the three shots. A few cameras can be set to bracket more than three shots per series.

For all three frames to be exposed with one shutter press, the release mode must be set to continuous low speed, continuous high speed, or self-timer. In other release modes, one shot will be taken each time the shutter-release button is pressed. It is on page 111 of the D7000 user manual.