memset(uart4.rx_buf, 0x00, sizeof(uart4.rx_buf));

This line of code initializes all the elements of the array uart4.rx_buf to the value 0x00. The function used here is memset(), which takes three arguments: the first argument is the pointer to the starting address of the memory block to be initialized, the second argument is the value to be set, and the third argument is the size of the memory block in bytes. Therefore, this line of code clears the data in uart4.rx_buf, which is likely used to store incoming data from a UART (Universal Asynchronous Receiver/Transmitter) communication port.