void OnRxDone( uint8_t *payload, uint16_t size, int16_t rssi, int8_t snr )

This is a function in a software program that is triggered when a radio receiver has successfully received data (indicated by the "RxDone" event). The function takes in four parameters:

1. A pointer to an array of bytes (uint8_t) that contains the received payload data
2. The size (in bytes) of the payload data
3. The received signal strength indicator (RSSI), which indicates the strength of the received signal
4. The signal-to-noise ratio (SNR), which is a measure of the quality of the received signal compared to the background noise level

The function would typically be used to process the received data and perform any necessary actions based on the contents of the payload. For example, it might decode a message and take appropriate action based on the message contents. The RSSI and SNR values can be used to evaluate the quality of the received signal and make decisions about how to handle future transmissions.