Substitution is a fundamental concept. Fortunately, it is also very simple and you can understand it because you use it ALL THE TIME!
Your screen name is "janetsroom". Is that what your mother calls you? Do your friends call you that? Your teachers?
If your name is Janet Stevens:
1) You or your friends might refer to yourself as "Jan",
2) Your mother, wishing to be more formal, might refer to you as "Janet", or
3) If she's angry may go for "Janet Stevens"!
4) A judge or teacher may refer to you as "Miss Stevens"
5) Your father may call you "Pumpkin" or something else cute.
6) I might call you "janetsroom", since that's all the information I have.
7) Some rotten little kid in your neighborhood may refer to you as "That mean girl that won't let me get away with anything."
Do you see what I'm getting at? There are or can be MANY names for the same thing. You are just one person. You are not unique in this regard. It applies to virtually everything.
If I know that y = Zebra, is it any different to say either of these:
x + y = 2
x + Zebra = 2
They are the same.
If y = 4, these are the same:
x + y = 6
x + 4 = 6
If y = "The Gettysburg Address", these are the same:
x - y = "Valley Forge"
x - ("The Gettysburg Address") = "Valley Forge"
All that is happening is one name or description is being substituted for another name or description.