You should know up front that you have the choice to not parti-gyle but say that you did. You would be a liar, of course, but you would still get to enjoy one of the benefits of parti-gyling as a homebrewer—that is, getting to say that you did it—without actually adding extra boils to your brew day or doing all the math.
Now that you have been advised, we will assume that you are an honest person and not a liar. Good for you! Let’s continue.
In short: Parti-gyling means getting multiple beers out of the same mash. The brewer boils successive runnings separately, and then, ideally, blends them to different strengths.