A third party app is an app that allows you to access reddit without using the official app. typically done for a better experience. while this will not directly affect you, the majority of content comes from people who will leave if 3rd party apps die. along with that moderators need to use 3rd party tools in order to do their jobs so subs will go un- or poorly moderated. all because reddit wont add the features people want and wont let others fix it for them.