Help me understand the process...

Hi, so I'm fairly new to this coding thing and I am currently on the Python track. I love it and some things I am grasping rather quickly while other things not so quickly. I picked up a book to supplement what I am learning via the videos, because sometimes I think some things are glossed over. I am currently reading Python Programming: An Introduction to Computer Science by John Zelle. It's really been helping. Right now I am reading the chapter on Strings, Lists, and Files. We did a little exercise where we take a Unicode-encoded message and have it convert to a string of text. I tried out the program and it worked well. What I need help on, is how exactly is it working. Here's the entire code:

def main():

    print("This program converts a sequence of of Unicode numbers into")
    print("the string of text that it represents.\n")

    inString = input("Please enter the Unicode-encoded message: ")
    message = ""
    for numStr in inString.split():
        codeNum = int(numStr)
        message = message + chr(codeNum)

    print("\nThe decoded message is:", message)

main()

Now I understand the input part, and I understand how the string is split. But I am a little lost once I get to this part:

        codeNum = int(numStr)
        message = message + chr(codeNum)

Can anyone break it down for me please? :)