Can't understand this codes process
It's easier to follow if you indent it properly. I'll do that, and add comments as I go:
void loop() {
    if ( Serial.available() &gt; 0 ) { // If there is something in the serial RX buffer

        static char input[inputLength]; // Create a working string to store data in
        static uint16_t i; // This is the current index in that working string

        char c = Serial.read(); // Read the next character from serial

        if ( c != '\n') // If it's not a &quot;line feed&quot; character...
            input[i++] = c; // then store it in the input string and increment i

        else { // Otherwise...
     
            input[i] = '\0'; // Terminate the string with a NULL to make it valid C
            i = 0; // Reset the index to 0 ready for next time

            uint16_t array[80]; // Create an 80-int array (temporary)
            uint16_t j = 0; // And an index variable

            if ( !strncmp(input, &quot;SEND&quot;, 4) ) { // If the string starts with SEND
                char* p = input + 4; // create a &quot;pointer&quot; to the string that starts 4 characters in

                while ( (p = strchr(p, ' ')) != NULL ) // While there is a space somewhere in the string
                    array[j++] = strtol(p, &amp;p, 16); // convert the first portion to a number, store it in the array, and advance the pointer to the next space.

                ir_start(array); // Send the array through IR
                Serial.println(&quot;WURKS&quot;); // Tell us it worked
            }
        }
    } 
}

So you see it receives data through serial and stores it, byte by byte, in input until it receives \n at which point it looks to see if it starts with SEND, then effectively &quot;cuts off&quot; the SEND by only considering everything after the SEND. Then it slices the remainder of the string up by looking for the next space each time and convert each portion to an integer which it stores in an array.
It then sends that array.