The HC-12 is a compact serial RF transceiver operating in the 433MHz ISM band. All intricacies of the analog and wireless side has been implemented and the only requirement is for the user to supply power and input/receive the data.
The modules uses the Silicon Labs Si4463 half duplex transceiver RF chip which has an SPI interface. This however is simplified further with an onboard STM8 microcontroller that does all the low level task and transforming the final interface to the user as a UART port.
In its factory default setting, the module is ready to be used to achieve transparent serial communications. Default baud rate is 9600. Solder the spring antenna into the ANT pad or supply your own antenna via the external antenna connector. The SET pin is only used when you need to do some configurations via AT commands. Otherwise, if you are happy with the default settings, you need only to provide 3.3V to 5V power at the Vcc + Gnd pins and send and receive data to and from the Rx and Tx pins.
Although Elecrow has provided a manual here, it does not appear to be the most updated version. There are however, numerous articles on this module online.