Login Items allows you to set applications to start automatically when you first log in to your Mac. They are used by many apps to run supporting apps in the background. You may have some items there added by apps and not realize it. You can also add your own apps. It is important to understand what the checkboxes next to the apps in the list do. If you want to stop an app from automatically launching, unchecking it won't do that. You need to remove the app from the list.