The code you attached is well written and well documented, but there's a lot going on, so if you are not familiar with C programming, it probably looks quite challenging. However, if you have worked with other programming languages before, you should be able to figure things out. I would suggest the following -
1. make a list of all of the functions in the program, and for each function, try to write an explanation for yourself of what it does, e.g.
unsigned int HEXtoASCII()
2. study the main() function - that is the function that controls what is happening in any C program. in this program, you can see the following sequence -
disable() - this disables interrupts
ActiveBoard = 4 - this selects which of the boards will receive commands
print ... - this prints the sequence "KC-4"
if you aren't certain what a function does, perhaps you can ask someone
3. finally, get a book on C programming. the classic book is "The C Programming Language" by Kernighan and Ritchie, originally published in 1978. it's the only C programming book I've ever owned, and C is my main programming language.
Good luck !