Soft Keys are buttons whose function changes depending on the context. They typically use part of the display to identify their current function. Because of this, they are usually located directly adjacent the display.
Soft keys are common on feature phones without touch screens. Smartphones generally have touch screens, and therefore don't need soft keys.
For example, it is common for basic phones to have two soft keys, located directly below the main display. At any given moment, the function of each soft key is indicated by corresponding text on the display. In the example in the photo, pressing the left soft key will activate the camera, while pressing the right soft key will open the contacts list. When the phone is in a different mode, the two soft keys have different functions.