As a publisher, you are trying to sell space on your sites and apps. This space is referred to as inventory, and an Ad Unit, within DFP, is the representation of that inventory you want to sell.
The Ad Unit is what will be targeted when the ad is being delivered, and, therefore, should be set up to appropriately categorize your apps, web pages, or portions of your web pages. We’ll get into ad delivery settings more later on, but if you, for example, have two different web pages that you want to serve the same ads to, you would only need to set up one ad unit in DFP that represents them both. Inversely, if you have one page that you want to serve very different ads on the top and bottom of the page, you might want to set up two ad units for that one page.