The Comparing Place describes a process that happens continuously in the brain. It is happening as you this. What you just experienced as you read the last sentence will help you understand the concept of The Comparing Place.
As we experience life, we are constantly comparing what we want (our Quality World pictures) with what we've got (our Perceived World). When the two match fairly well, we feel good. When there is a mismatch, as there probably was when you read the sentence above, you feel a degree of frustration, depending on how important the Quality World picture is to you. That frustration signal, as Glasser terms it, is felt as an urge to behave in a way that will help us get more of what we want.
As you read the sentence above, you probably felt a slight frustration signal when you read "It is happening as you this." What you probably want (your Quality World picture) when you read something is for it to make sense. Since the word "read" is missing from the sentence, you may have experience a frustration signal which may have urged you to go back and reread the sentence. Once you did that, you probably realized a word was missing and your brain supplied a word that would make sense in that sentence. Once you did that, the frustration signal disappeared.
Glasser likens the Comparing Place to a set of scales. When your scales are in balance, when what you want is what you have, you continue to do what you've been doing. When your scales get out of balance, you feel the frustration signal, the urge to behave.