Probe in the driver gets called whenever a device's (already registered) name matches with the name of your platform driver (here, it is bcm-gpio). The second major functionality is ioctl which acts as a bridge between the application space and your driver. In technical terms, whenever your application invokes this ( ioctl) system call, the call will be routed to this function of your driver. Once the call from the application is in your driver, you can process or provide data inside the driver and can respond to the application.
The SoC datasheet, i.e., BCM2835-ARM-Peripherals, plays a pivotal role in building up this driver. It consists of all the information pertaining to the peripherals supported by your SoC. It exposes all the registers relevant to a particular peripheral, which is where the key is. Once you know what registers of a peripheral are to be configured, half the job is done. Be cautious about which address has to be used to access these peripherals.