Your protagonist must undergo the process of developing a personality, must possess quirks and flaws, and must be able to act or react “in character” whenever appearing in your story. Some of the characters in your story, however, are not expected to do this. These are the ones who appear only for the purpose of moving the story along in a rational way. In other words, they don't perform any specific role, they only perform tasks that make everything move along in a natural way.
Let's say your protag is walking down the street and passes the mail carrier. A friendly "Hello" would be expected, so when that character speaks or nods, he/she is being used only to keep the reality in line. Your protag will be expected to react to that character in some way. Is your protag deep in thought? Is he/she trying to maintain a low profile? If either of these is the case, the protag may not react by returning the nod or uttering a reply. Instead, he/she may lower his/her head and pass without acknowledging the mail carrier. If your protag is happy and loving life at that moment, he/she may respond by blurting out, "Hello! Isn't it a wonderful day?" Either way, your reader is being fed some information about your protag at that particular time in the flow of the story.
Whatever the response, that mail carrier has provided an avenue for the reader to get to know the protag's current state of emotion and has, therefore, provided a piece of information without being an important character. And that's what is called a flat character. He/she has been important only at that precise moment but will probably not be seen or heard from again throughout the book. Flat characters can be compared to cutout paper dolls in that they will not take up much space in the story. They are, quite frankly, easily disposed of after they do their "bit part."