What does Softkey mean?
A softkey is a key on a device which can have context-sensitive or user-programmable functions, but generally it just means it has more than one function. Unlike letters on the keyboard and number keys on cell phones which cannot be reprogrammed and are therefore considered as hard keys, softkeys can change function. One example of softkeys is the keyboard's function or F-keys which have different specialized functions depending on the application and context.
Techopedia explains Softkey
Softkeys are found on many devices and are an essential feature to make the device more customizable and user friendly. For example, cell phones predating the rise of smartphones usually had two softkeys located above the call and cancel keys, which served different purposes depending on the current application or menu. Sometimes one of the keys was used for deleting and at other times for further navigation. Some cell phones even made these keys user-programmable, where they could act as shortcuts to open a specific application on the phone.
Alternatively, softkeys have taken on a new meaning in the world of touchscreen devices. They are sometimes used to refer to the software or touchscreen keyboard. Manufacturers refer to these as "soft key keyboards," alluding to their nature as software-generated keys rather than being reprogrammable. But because of the openness of mobile operating systems for developers, the hardkeys of a phone can become softkeys through third-party applications, such as those that turn the volume keys or the power button into the camera shutter button.