Okay kiddo, imagine you are building a tower out of blocks. Now let's say you built a base out of 4 big blocks, and on top of those big blocks you added 2 smaller blocks. The smaller blocks are on top of the big blocks, right?
That's kinda like what upper structure means. It's like you have a base foundation made up of simple chords, and then on top of that you build more complex chords.
So let's say your base foundation is a simple C major chord. You might add some notes on top of that to make it more interesting or complex, and those added notes make up the upper structure.
Does that make sense, little one?